Debian/Bullseye plasma screen doesn't lock

[Gary Dale]

Since mid-August, my screen doesn't lock automatically after 10 minutes 
(as I specified in the power management settings). I've tried rebooting 
and resetting the power management setting but it's still not locking. 
This has survived several updates to the window management software.

I tried sending this issue to the debian-kde list but it hasn't gotten 
through.

I'm running Debian/Bullseye on an AMD64 system.

Any suggestions on how to fix this?