Build and run the attached test app against Qt 6.0.0. It registers an action with shortcut 'Alt-§'. This is the top-left-most key on the keyboard on my UK keyboard.

Then press Opt-§ to trigger the action. It crashes in QCocoaKeyMapper::keyMapForKey when comparing the results of the Cocoa and Carbon APIs. The immediate cause is indexing into an empty string, but perhaps this suggests something wrong with the way the newer API is being used anyway.