Hello,
I use the french variant keymap «Bépo» and I have a problem with the “_” symbol (underscore) which is not functional with his key combination “AltGr + space”.
I use Manjaro+KDE and I checked in console mode (Ctrl+Alt+F2, then $sudo loadkeys fr-bepo) and it gives me the desired character! So KDE has to do something that breaks this!
If you have an idea I’m interested.

Well, by testing with LXQT the problem disappeared! I don't know exactly why, but after installing LXQT I found a lot of conf RAZ on KDE side.
In short, auto-resolution without knowing the origin of the problem ... too bad.