The problem is that the compose key works fine, but some application doesn't accept some characters from it. E.g. I can type the character
∞ in Emacs (Compose+8+8), but this won't work in FireFox, Konsole and Kate. But many other characters, e.g.
€ typed there (in FireFox, Konsole and Kate) with Compose work just fine. Also I may insert a problem symbols with a simple copy-paste (from any of two clipboards).
So, what could cause it, and how to fix it?
Sorry, but I have no idea how to research the issue. I found people whose Compose key didn't worked at all in some apps, but in my case it is works, though somehow partially.
Setup: the compose key bound to the right Super key in both
/etc/default/keyboard and KDE keyboard settings.