I have just posted a new topic here about this but found this thread after posting it.
I have two xubuntu 20.04 systems, one has the problem and the other doesn’t! 
Unsetting GTK_IM_MODULE fixes the issue for me OK.
I’ve looked for obvious differences between the two systems but at the moment I can’t see anything related to GTK_IM_MODULE. The script that sets it X11/Xsession.d/70im-config_launch is present on both systems. I’m quite happy to dig around further in the nether regions of /etc if anyone has any good ideas of what to look for.