KDE 5 has "some issues" handling an external monitor plugged to the laptop.
While it remembers the resolution and relative position, the laying out of the panels isn't very smart. 

Reproducible: Always

Steps to Reproduce:
1. Plug an external monitor to your laptop
2. Add a panel with a task manager to the new screen
3. Move this panel to the small laptop's screen, and the panel from the laptop screen to the external monitor (this is sometimes necessary because when you disconnect the monitor, its panel is moved to the other screen, overlapping with the other one. After reconnecting the monitor, it's not always the case, that the overlapping panel is automatically moved back to the screen)


Actual Results:  
Task manager on screen A shows only the tasks from screen B and vice versa.

Expected Results:  
Task manager A showing tasks on screen A