SUMMARY
Window rules "Apply Now" and "Apply Initially" do not appear to be working. I've tried rules with 2 different applications, and neither one are having the expected affect. The "Force" option did work, but then rendered the desktop and the activity unusable until after I logged out and back in.

STEPS TO REPRODUCE
1. Create a new Windows Rule with the "Apply Initially" or "Apply Now" setting.
2. Apply the rule

OBSERVED RESULT
Rule does not apply

EXPECTED RESULT
Rule applies

SOFTWARE/OS VERSIONS
Operating System: Kubuntu 21.04
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
Kernel Version: 5.11.0-16-generic x64

ADDITIONAL INFORMATION
At first, the rules didn't even save, so there may be an underlying issue here.

Sample Rule:
Description: Thunderbird
Window class (application) Exact Match thunderbird
Match whole window class No
Window types All selected
Size & Position
Activities Apply Now System

The screen does "flash black" when the rule is run.

Why do you have Frameworks 5.80.0? Kubuntu 21.04 ships Frameworks 5.81.0 by default...

I don't know why. I did not manually downgrade it. I also haven't added any software sources in Discover, so that shouldn't be a problem either. Discover says everything is up to date, so I'm not sure what the problem is.

Do you have any ideas? Do you know what software source the Frameworks would be part of? I'm not very familiar with the Kubuntu packaging sources.

Update: I checked the live image I downloaded from https://kubuntu.org/ and it has Frameworks Version 5.80.0 as well. Also, with Discover saying I am up to date, I expect that 5.80.0 is the latest version in the Kubuntu packages right now.

yes, my bad, sorry, I have the backports PPA enabled and forgot about.

No problem at all.

Update: All of my window rules are gone again. I guess something didn't like them, and deleted them.