Notice that for Win32 to work, you need to alter (or create) your .exe manifest, otherwise Windows will not inform the correct values (there are 7 UI libraries in Windows that deal with HDPI differently, 3 of them do not support the concept at all (GDI, GDI+ and MFC)).
I’m not sure, but I think Windows Flutter is a Win32 app (not a UWP one), so, in order to get the correct values, you should create a your_app.exe.manifest file with the correct settings so the calls are redirected to the correct ones.