SUMMARY
I have a 3-screen setup consisting of one notebook (left) and 2 external screens (center, right). Each screen has a panel with a task manager that only shows the tasks from the specific screen, desktop and activity.
When I close the notebook, the respective screen is deactivated and the windows are moved to the center screen, but are not shown in the Task Manager. 
When I open the notebook up again, windows are moved back, and are shown in the left screens task manager, however, the center screen task manager is now *empty*, not even showing the center screen tasks.
This can be fixed by moving the panel to the right screen and back.

STEPS TO REPRODUCE
In a 3-screen setup:
1. Create Task Managers on each screen, only showing that screens/desktops/activities tasks.
2. Start tasks on all three screens.
3. Deactivate left screen (e.g. close notebook).
4. Activate left screen.
5. Move center screens task bar to right screen and back.

OBSERVED RESULT

After step 3, the windows from the deactivated left screen will show up on the center screen, but are not represented in the center Task Manager.

After step 4, the windows are back on the left screen, but the center Task Manager is empty.

After step 5, everything is back to normal again.

EXPECTED RESULT
Task Manager should reflect the tasks on the respective screens all the time.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 20.10
KDE Plasma Version: 5.19.5
KDE Frameworks Version: 5.74.0
Qt Version: 5.14.2

ADDITIONAL INFORMATION
This has been going on for a few Ubuntu versions now, it does not seem to be 20.10 specific.