One more note about Kwin Rules:
Currently, and this is true for many small things in KDE, Kwin Rules work better in KDE X11 sessions over KDE Wayland sessions (which is default now).
I was in a Wayland session for this weekend thinking why my window rules weren’t working every time windows that have specific rules would pop up under my cursor instead of centered, to the right, etc.
On an unrelated note, my KDE Wayland session froze for about 15 seconds while using the Krita AppImage, and I thought, “This never happens in Openbox.” So, I decided to check if it was Wayland-related.
Lo and behold, I started a KDE X11 session and my window rules are all working as expected, with only windows that don’t have rules opening under my cursor.
So, in case you run into an issue where a rule just seems to not work, it might just be a Wayland issue that will be ironed out eventually.
You can only practice patience in this case.
But also, report it if you have the time or care to do so.