Does this make a difference?

$csv=foreach ($policy in $PolicyObjects) {
$GPO = Open-NetGPO -PolicyStore "contoso.com\$policy"
$policy | Out-File $env:temp\gpos.csv  -Append -Force
Get-NetFirewallRule -GPOSession $GPO  | 
Select Name,
DisplayName,
DisplayGroup,
@{Name='Protocol';Expression={($PSItem | Get-NetFirewallPortFilter).Protocol}},
@{Name='LocalPort';Expression={($PSItem | Get-NetFirewallPortFilter).LocalPort}},
@{Name='RemotePort';Expression={($PSItem | Get-NetFirewallPortFilter).RemotePort}},
@{Name='RemoteAddress';Expression={($PSItem | Get-NetFirewallAddressFilter).RemoteAddress}},
Enabled,
Profile,
Direction,
Action 
}
$csv | Export-CSV $env:temp\gpos.csv -Append -Force
Start-Process notepad $env:temp\gpos.csv 

It's not possible to append data with Export-Csv when the target file does not match the structure.

In your case, Export-Csv tried to append values from column $policy, which doesn't exist. Therefore it added $null.

For a valid CSV file you have to combine everything into one object before using Export-Csv.

Try the below approach, which should work much faster, since it does not use Get-NetFirewallAddressFilter several times for the same item, but only once for per GPO-Session.

Unfortunately I can't test it with a GPO-Session, but since the Parameter -GPOSession exists for Get-NetFirewallPortFilter and Get-NetFirewallAddressFilter as well, it may work. If you want to test it just locally, you just need to remove the outer foreach-loop and all GPOSession related commands and parameters. Open-NetGPO and -GPOSession

Comparison between my approach and retrieving the Port- and Address-Filter for each rule separately (even when only once per rule) when building a report for local firewall rules:

My approach: < 2 seconds
Other : > 3 minutes (using Get-NetFirwall*Filter for each rule once)

The code requires an elevated console!

#requires -RunAsAdministrator
$ruleReport = [System.Collections.Generic.List[psobject]]::new()
foreach ($policy in $PolicyObjects) {
    <#
        if you want to append to file after each policy, place the list here instead of above
        $ruleReport = [System.Collections.Generic.List[psobject]]::new()
    #>

    $GPO   = Open-NetGPO -PolicyStore "contoso.com\$policy"
    $rules = Get-NetFirewallRule -GPOSession $GPO

    # retrieving filters once in advance speeds up the execution extremely! (Requires elevated session to get all filters)
    # local execution time with that approach reduces the execution time from   >3 minutes  TO   <2 seconds
    $portFilters    = Get-NetFirewallPortFilter -All -GPOSession $GPO
    $addressFilters = Get-NetFirewallAddressFilter -All -GPOSession $GPO

    # build dictionary for port filters for rule lookups
    $portFilterDict    = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
    foreach ($portFilter in $portFilters) {
        $portFilterDict.Add($portFilter.InstanceID, $portFilter)
    }

    # build dictionary for addresss filters for rule lookups
    $addressFilterDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
    foreach ($addressFilter in $addressFilters) {
        $addressFilterDict.Add($addressFilter.InstanceID, $addressFilter)
    }


    foreach ($rule in $rules) {
        $ruleInstanceId    = $rule.InstanceID
        $rulePortFilter    = $portFilterDict[$ruleInstanceId]
        $ruleAddressFilter = $addressFilterDict[$ruleInstanceId]

        # build combined object
        $ruleReportItem = [pscustomobject][ordered]@{
            Policy        = $policy
            Name          = $rule.Name
            DisplayName   = $rule.DisplayName
            DisplayGroup  = $rule.DisplayGroup
            Protocol      = $rulePortFilter.Protocol
            LocalPort     = $rulePortFilter.LocalPort
            RemotePort    = $rulePortFilter.RemotePort
            RemoteAddress = $ruleAddressFilter.RemoteAddress
            Enabled       = $rule.Enabled
            Profile       = $rule.Profiles
            Direction     = $rule.Direction
            Action        = $rule.Action
        }

        # append to list
        $ruleReport.Add($ruleReportItem)
    }

    <#
        if you want to append to file after each policy, place the export here instead of below
        $ruleReport | Export-Csv $env:temp\gpos.csv -Append
    #>
}

$ruleReport | Export-Csv $env:temp\gpos.csv

Local Firewall Rules

To test the approach just for local Firewall Rules, use this script.
This script adds all available information of each rule and requires an elevated console.

#requires -RunAsAdministrator
$ruleReport = [System.Collections.Generic.List[psobject]]::new()

$rules                = Get-NetFirewallRule
$portFilters          = Get-NetFirewallPortFilter -All
$addressFilters       = Get-NetFirewallAddressFilter -All

$applicationFilters   = Get-NetFirewallApplicationFilter -All
$interfaceFilters     = Get-NetFirewallInterfaceFilter -All
$interfaceTypeFilters = Get-NetFirewallInterfaceTypeFilter -All
$securityFilters      = Get-NetFirewallSecurityFilter -All
$serviceFilters       = Get-NetFirewallServiceFilter -All


$portFilterDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $portFilters) {
    $portFilterDict.Add($filter.InstanceID, $filter)
}

$addressFilterDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $addressFilters) {
    $addressFilterDict.Add($filter.InstanceID, $filter)
}

$applicationFiltersDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $applicationFilters) {
    $applicationFiltersDict.Add($filter.InstanceID, $filter)
}

$interfaceFiltersDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $interfaceFilters) {
    $interfaceFiltersDict.Add($filter.InstanceID, $filter)
}


$interfaceTypeFiltersDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $interfaceTypeFilters) {
    $interfaceTypeFiltersDict.Add($filter.InstanceID, $filter)
}


$securityFiltersDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $securityFilters) {
    $securityFiltersDict.Add($filter.InstanceID, $filter)
}


$serviceFiltersDict = [System.Collections.Generic.Dictionary[[string], [ciminstance]]]::new()
foreach ($filter in $serviceFilters) {
    $serviceFiltersDict.Add($filter.InstanceID, $filter)
}




$cimClassProperties = 'CimClass', 'CimInstanceProperties', 'CimSystemProperties'
$cimClassPropertiesLookup = [System.Collections.Generic.HashSet[string]]::new([System.StringComparer]::OrdinalIgnoreCase)
foreach ($cimClassProperty in $cimClassProperties) {
    [void] $cimClassPropertiesLookup.Add($cimClassProperty)
}


foreach ($rule in $rules) {
    $ruleInstanceId      = $rule.InstanceID
    $portFilter          = $portFilterDict[$ruleInstanceId]
    $addressFilter       = $addressFilterDict[$ruleInstanceId]
    $applicationFilter   = $applicationFiltersDict[$ruleInstanceId]
    $interfaceFilter     = $interfaceFiltersDict[$ruleInstanceId]
    $interfaceTypeFilter = $interfaceTypeFiltersDict[$ruleInstanceId]
    $securityFilter      = $securityFiltersDict[$ruleInstanceId]
    $serviceFilter       = $serviceFiltersDict[$ruleInstanceId]

    $ruleReportItemProps = [ordered]@{
        RuleDisplayName            = $rule.DisplayName
        RuleDescription            = $rule.Description
        RuleEnabled                = $rule.Enabled
        RuleProfile                = $rule.Profile
        RuleAction                 = $rule.Action
        RuleDirection              = $rule.Direction
        PortFilterProtocol         = $rulePortFilter.Protocol
        PortFilterLocalPort        = $rulePortFilter.LocalPort
        PortFilterRemotePort       = $rulePortFilter.RemotePort
        AddressFilterRemoteAddress = $ruleAddressFilter.RemoteAddress
        ApplicationFilterProgram   = $applicationFilter.Program
    }

    foreach ($prop in $rule.psobject.Properties) {
        $propName = 'Rule' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName ) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName , $prop.Value)
        }
    }

    foreach ($prop in $portFilter.psobject.Properties) {
        $propName = 'PortFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    foreach ($prop in $addressFilter.psobject.Properties) {
        $propName = 'AddressFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    foreach ($prop in $applicationFilter.psobject.Properties) {
        $propName = 'ApplicationFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    foreach ($prop in $interfaceFilter.psobject.Properties) {
        $propName = 'InterfaceFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    foreach ($prop in $interfaceTypeFilter.psobject.Properties) {
        $propName = 'InterfaceTypeFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    foreach ($prop in $securityFilter.psobject.Properties) {
        $propName = 'SecurityFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    foreach ($prop in $serviceFilter.psobject.Properties) {
        $propName = 'ServiceFilter' + $prop.Name
        if (-not $ruleReportItemProps.Contains($propName) -and -not $cimClassPropertiesLookup.Contains($prop.Name)) {
            $ruleReportItemProps.Add($propName, $prop.Value)
        }
    }

    $ruleReport.Add([pscustomobject]$ruleReportItemProps)
}

$ruleReport | Sort-Object RuleEnabled, RuleDisplayName, RuleProfile | ogv