To follow up: this problem can be fixed by adding this:
apc.enable_cli=1
to /etc/php7.4/cli/conf.d/20-apci.ini. As mentioned here.
In summary: everything is working now, but these issues were unexpected for a 21.0.3 release, i.e. after several months of deploymeny and a few revisions.
Suggestion: perhaps consider adding extra tests to the upgrade scripts to catch this?