Alt-Key stopped working in gnome-terminal after upgrade to Ubuntu 19.04

For example I cannot switch between the tabs in gnome-terminal with Alt+1, Alt+2 anymore. Shortcuts in general and Menu shortcuts specifically, are enabled via the global gnome-terminal settings.

When I hit Alt + 1 it looks like this, and it’s the same behaviour without tmux:

enter image description here

I already opened an issue in the gnome-terminal repo, but it was rejected. It really decreases the usability of the gnome-terminal for me.

I’d be happy to find any solution to that, even if it requires a bit of effort.

Plasma: alt+key not working

After upgrading Kubuntu to 19.04, my alt key isn’t working in combination with other keys (like alt+tab). I need to enter settings -> input devices -> keyboard and change any option, after pressing “apply”, the keyboard works fine again. I have to do this every time I reboot my notebook. With xev and showkey I can see that the keypress is being recognized, but I don’t know what else can I see to find what’s the problem