I found it… but it is suffering of the same bug like the Mate version.
I set the default calendar to Evolution and after the next login it was visible, but only for a few moments. A few seconds later, it disappears again. I had this when using Mate too. Came out of the blue.
There was a thread I found about this, but no solution yet. Seems it is not Mate-related then, since the bug is present in this one too then.
Obviously it crashed. I found this thread https://askubuntu.com/questions/1341333/indicator-datetime-service-crash-ubuntu-20-04
I checked my /var/crash/ directory and indeed it crashed.
ProblemType: Crash
Architecture: amd64
CrashCounter: 1
CurrentDesktop: Budgie:GNOME
Date: Sun Oct 3 20:09:53 2021
DistroRelease: Ubuntu 20.04
ExecutablePath: /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service
ExecutableTimestamp: 1581950243
ProcCmdline: /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service
But again, thank you for this. Maybe we can even get it working somehow 
Edit: This https://ubuntu-mate.community/t/indicator-applet-complete-stopped-showing-the-clock/23931/15 is the thread where I first learned about the same bug while using Mate. Who knows, maybe it is of some help to you.