If you pass a file path to the 'dir' argument of the function QFileDialog::getSaveFileName but the file doesn't exists (e.g. /home/<user>/newFile.txt), then the initial selected file name is not set.
I checked and the problem is only reproducible using the gtk2 plugin.
To reproduce it you can modify the standarddialogs example and change line dialog.cpp:422 so that you pass to the function an file path that doesn't exist.
With version before 5.3.0 it was working correctly.
The file is only reproducible with the gtk2 plugin and not with the default Qt dialog.