First, open Chromium Devtools and start logging keydown events with:

document.addEventListener("keydown", function(event) { console.log(event); }, true); 

Then, using a German keyboard layout, try typing (for example) AltGr + M.

When using Qt WebEngine, the event's ctrlKey field is erroneously marked as true. In addition, Qt seems unable to distinguish between left-Alt and right-Alt. 

I've attached screenshots of what the event log looks like for AltGr + M in Chrome (80.x) versus Qt Webengine (running the Simple Browser example).

Unfortunately, the behavior in Qt makes it impossible to distinguish between a user typing Ctrl + Alt + M from AltGr + M.