This bug is probably a pure Qt bug, unlike the Ubuntu-only issue with appmenu-qt5 which affected keyboard shortcuts within the application, not related to native modal dialogs like the file picker or to context menus for text input fields.
Actually, this seems to be more complicated and Jan was very much on it asking for shortcuts shown in the Edit menu, because commenters in the bug report mention that on Mac an application has to provide standard menu items with standard shortcuts to make them work in the native QFileDialog with fixed Qt version! Unfortunately, riji misunderstood the request and attached a wrong screenshot.
As far as I understand, Avidemux uses a custom menu, and I have no idea which implications this may have in respect to QFileDialog.
riji, would you attach a screenshot of the Edit menu in Avidemux on Mac, please?