On my Linux Mint 19.3 - Cinnamon, I have observed that sometimes the compose-key works, sometimes it doesn't. Especially, it stopped working after reboot.
Investigating it led me to the conclusions that
- it does NOT ALWAYS stop working after reboot.
- it comes back after switching back from a v-term (Ctrl-Alt-F1, then [Ctrl-]Alt-F7)
- Killing (and auto restarting) cinnamon-session doesn't seem to make problem re-appear
a friend of mine tested it on his Ubuntu 20.4 - Mate and didn't observe this problem. I don't know if the problem might be related with Linux Mint or with Cinnamon
Any lead on what else I could try to investigate this ?