I have a working keyboard setup in my
.xkbmap and make it active by
xkbcomp $HOME/.xkbmap $DISPLAY. But it suddenly stopped working. Somehow it does not change keyboard settings. I am totally confused.
I am using Debian/testing with Gnome.
It's really helpfully in questions if instead of "suddenly it stopped working" you actually describe the circumstances, as an "after my recent upgrade to xwayland".
While Wayland uses
xkb, it works in a different way from X, and details depend on the compositor.
So if you are doing
xkbcomp $HOME/.xkbmap $DISPLAY,
xkbmap uses the X protocol to contact XWayland, which promptly ignores it because there's no way to change keyboard mappings in Wayland proper while running. So your upgrade to XWayland broke things (which is not really surprising, considering they are completely differet systems), and it didn't "suddenly stopped working".
This question explains how to customise your keyboard mappings with wayland. Basically you need to change (or better: add to) the system XKB database.