My Mac mini (2018) is failing to shut down from the menu option

My Mac mini (2018) is failing to shut down from the menu option. I have to press and hold the power button for more than ten seconds for it to properly shutdown.

Normally this shutdown reason is reported (for me) as "-11", which I don't see listed as a valid code. What does it mean?

Presumably something is running that prevents it from shutting down, but how do I find out what it is? The screen goes blank and the power light turns off, but the machine hasn't completely stopped, the only way I can tell is that it stays warm permanently until I use the button to shut it down.

However, today I restarted it and the last shutdown reason is given as "-20", which is apparently to do with the T2 chip. Does this give a clue as to what could be preventing it shutting down?

[Re-Titled by Moderator]