Checked on Ubuntu x64 (with its GTK based Qt5) and Fedora (KDE Plasma)
29. In my app qt5 build, OpenDialog and SaveDialog file filters are empty (while they are set in app). Confirmed?

Sorry, that Filter works ok.

Another question. In qt5/qtwsdialogs.pp we have

   if (AFileDialog is TSaveDialog) and (trim(TmpFilter)='()') then
     Result := ''
   else
     Result := GetUtf8String(TmpFilter);

so that empty dlg.Filter will give empty result Qt filter. Only for SaveDialog! Why only for SaveDailog? it is needed for OpenDialog too, because result "()" looks bad.

AT