Sorry, I somehow missed this.
My current understanding is that AltGr is read as Ctrl+Alt unless QT_QPA_PLATFORM=windows:altgr is set, and both can be used to access the third level of a key, unless there’s an equivalent shortcut with higher precedence. This issue doesn’t seem to fit with that and I can’t reproduce it, regardless of the environment variable and keyboard layout.
Which version are you using, @LG_GR, and can’t you use the AltGr key?