ASP.NET Custom Printing using Javascript Not retreiving DropDownList Value

We have a custom print button on an aspx page. The button is using the following code to open a new page with the print dialog. All elements are appearing correctly in the print preview except the dropdownlist that has a lot of items. Other dropdownlist selected values are appearing correctly but they have 5-6 items only.

However if I print->cancel->print , the second time the preview print page is displayed, the selected value of the drop down is displayed correctly. Ctr+P works correctly too but it prints the entire page (not required). This issue is same for chrome/firefox/edge

    <script type="text/javascript">
                function CallPrint(strid) {
                     var WinPrint = window.open('', "", 'width=1500,height=1500');
                     var prtContent = document.getElementById(strid);
                         WinPrint.document.write(prtContent.innerHTML);
                         WinPrint.document.write('<link rel=stylesheet href=StyleSheet.css>')
                         WinPrint.document.close();
                      WinPrint.onload = function () { // wait until all resources loaded 
                      WinPrint.focus();
                      WinPrint.print();  
                      WinPrint.close();
                };    
                     setTimeout(window.close, 500);
                return true;
            }
       </script>

part of the aspx

 <div id="print">  ......
                    <td class="auto-style162">
                        <ajaxToolkit:ComboBox ID="dd_aj" runat="server" AutoCompleteMode="Suggest" DataSourceID="sql_aj" DataTextField="Job_No" DataValueField="Job_Key" DropDownStyle="DropDownList" MaxLength="0" Style="display: inline;">
                        </ajaxToolkit:ComboBox>
                    </td> .....
<asp:Button ID="BtnPrint" runat="server" Text="Print" OnClientClick="javascript:CallPrint('print');" CssClass="alert info" />
  </div>

Same result with a standard (non ajaxtoolkit) dropdown list.

I am aware of using CSS Media print, but we cannot use that for now.

I have tried using the setTimeout and looked at other threads here like asp.net Printing Page using javascript

I am suspecting that since the dropdown has many items it takes too long to render.