Unable to compose characters on Mac OS in text edits

In a QPlainText or a QLineEdit I am unable to compose the character ü by pressing ALT + u (which normally gives me only the dots ¨, highlighted yellow) followed by u.

Instead, pressing ALT + u yields nothing and the next keypress is swallowed.

On a debug console of my application I see the following:

 qt.qpa.keymapper: Mismatch between Cocoa '\u00a8' and Carbon '\x0' for virtual key 32 with QFlags<Qt::KeyboardModifier>(AltModifier)

Please let me know if you need more information about my environment.

I am on Mac OS Big Sur 11.5.1 on a Apple M1, but building for x86_64, and I am using Qt 6.2.0 (beta) downloaded and installed via installer on 31.07.2021.

I found this old bug report, but is was closed with no solution: QTCREATORBUG-12372

I dug around a bit more and found the corresponding source file on the 6.2 branch of qtbase: https://github.com/qt/qtbase/blob/fe83bc697f09da5eaeeb9179f2dd5abd9ff2f91d/src/gui/platform/darwin/qapplekeymapper.mm#L536

I have no idea, why carbon and cocoa are contradicting each other, and if the composition should not happen somewhat before all the Qt event handling?