Laravel SQL missing data from Join table

I'm using the following code to create a query with a join.

$query      =   rs_vehicles::select(   
                            'rs_vehicles.id',
                            'rs_vehicles.type', 
                            'rs_vehicles.make', 
                            'rs_vehicles.model',
                            'rs_vehicles.year',
                            'rs_vehicles.status',
                            'rs_vehicle_regs.registration'

                            );            

    $query      ->leftJoin('rs_vehicle_regs', function($join){

                    $join->on('rs_vehicle_regs.rs_vehicles_id', '=', 'rs_vehicles.id')
                         ->where('rs_vehicle_regs.registration_date', '=',
                                DB::raw("(select max(registration_date) from rs_vehicle_regs where rs_vehicles_id = rs_vehicles.id)"));
                });       

    $rsGrid =   $query->get();

This produces the following sql statement:

select rs_vehicles.id, rs_vehicles.type, rs_vehicles.make, 
s_vehicles.model, rs_vehicles.year, rs_vehicles.status, 
s_vehicle_regs.registration from rs_vehicles 
left join rs_vehicle_regs 
on rs_vehicle_regs.rs_vehicles_id = rs_vehicles.id 
and rs_vehicle_regs.registration_date = (select max(registration_date) from rs_vehicle_regs where rs_vehicles_id = rs_vehicles.id)

The SQL statement generated by Laravel runs perfect when I execute it in MySQL Workbench and it brings back the expected values for ALL fields. However when I execute the code in Laravel although the script runs perfectly with no errors and brings back almost all the required data the 'rs_vehicle_regs.registration' field which is in the joined table comes back as null. I've spent ages trying to figure this out and I am getting nowhere. Anybody any idea why this wont work in Laravel? I'm at my wits end.

I have tried changing get() to toSql() and I get the following

select `rs_vehicles`.`id`, `rs_vehicles`.`type`, `rs_vehicles`.`make`, `rs_vehicles`.`model`, `rs_vehicles`.`year`, `rs_vehicles`.`vehicle_status`, `vh_type`.`display_text` as `type_text`, `vh_status`.`display_text` as `vehicle_status_text`, `rs_vehicle_regs`.`registration` as `registration` from `rs_vehicles` left join `app_params` as `vh_type` on `rs_vehicles`.`type` = `vh_type`.`list_value` and `vh_type`.`param_type` = ? left join `app_params` as `vh_status` on `rs_vehicles`.`vehicle_status` = `vh_status`.`list_value` and `vh_status`.`param_type` = ? left join `rs_vehicle_regs` on `rs_vehicle_regs`.`rs_vehicles_id` = `rs_vehicles`.`id` where `rs_vehicles`.`status` = ?

I have also tried the following:

        $query      ->leftJoin('rs_vehicle_regs', function($join){
                $join->on('rs_vehicle_regs.rs_vehicles_id', '=', 'rs_employees.rs_vehicles_id')
                     ->where('rs_vehicle_regs.registration_date', '=', 'max(registration_date)'); 
            });    

This produces the exact same result as the original query where the object as the field but the values are null.

Then I tried this:

$query      ->leftJoin('rs_vehicle_regs', 'rs_vehicle_regs.rs_vehicles_id', '=', 'rs_vehicles.id');

This join works and brings back the data I am looking for however if the join table has more than one entry related to the parent table as can happen I get all records returned and I only want one record were max(registration_date). This has me totally stumped.