SUMMARY
I have a 2560x1440 monitor and a 1920x1080 tv as second monitor.

          -----------------
--------- |               |
|       | |    primary    |
|  tv   | |    monitor    |
--------- |               |
          -----------------

With one monitor size and position is respected when using
"allow kde apps to remember the position ..." setting
or window rules (remember size and position).

With two monitors and "allow kde apps to remember the position ..." setting
and/or window rules, if I close the app in the left monitor,
it opens in the left screen with the size and position it had when closed.

But if I close it in the right monitor it doesn't,
it opens in the correct position but with wrong size;
very small 640x514.

Tested app was kate.

STEPS TO REPRODUCE
1a. Enable "allow kde apps to remember the position ..." setting
1b. Setup window rules size and position to remember
2. Open app
3. Resize and reposition
4. Close app
5. Reopen app

OBSERVED RESULT
App opens with wrong size, if closed in right monitor.

EXPECTED RESULT
App opens with the size it had when last closed,
regardless in which monitor it was closed.

Operating System: openSUSE Tumbleweed 20201025
KDE Plasma Version: 5.20.1
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1
Graphics Processor: Nvidia GTX 750 Ti