Earlier quoted context omitted.
It's not a Wayland problem, ibus does not work in KDE even on X11.
Uh? I've been using ibus with mozc for Japanese input on KDE for the past 3 years.
In my case, I wanted to keep my configured xkb keyboard layouts + I wanted to be able to use ibus in parallel for CJK.
Ok, there must be a problem with *my* KDE setup: when I try to disable KDE keyboard layouts, KDE does not persist changes to the next session: when I log in next time, KDE keyboard layouts are enabled again and interfere with ibus.