This issue was previously more widespread, and was mostly fixed in 5.9. Please see the original report for background:
https://bugreports.qt.io/browse/QTBUG-54755

While characters seem to work properly on a normal keyboard now, application shortcuts are still stealing the characters when they are input with the AltGr key. If you change the repro case in the old bug report to detect a "@" character, typing "@" with a US keyboard layout will correctly input the character into the input box. But if you switch to a de-DE or de-CH layout and input the "@" character by pressing AltGr+2, the application shortcut is triggered instead.