Peculiar and inconsistent behaviour of compose-key after reboot. How should I troubleshoot?

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 1. it does **NOT ALWAYS** stop working after reboot. 2. **it comes back after switching back from a v-term** (Ctrl-Alt-F1, then [Ctrl-]Alt-F7) 3. 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 ?