When I connect a second monitor to my laptop the panel is not displayed on the primary output.

Reproducible: Always

Steps to Reproduce:
1. Configure dual monitor setup with laptop as primary output (1920x1080), external monitor (3840x2160) above laptop screen.
2. Boot laptop with external monitor disconnected -> Panel is at bottom of laptop screen
3. Connect external monitor

Actual Results:  
The panel is now displayed at the bottom of the external monitor although that is not the primary output. Also, when I open the application launcher menu it is displayed not at the bottom left but at the middle left of the screen. For displaying the menu a screen y resolution of 1080 seems to be used, which is correct for the primary (laptop) screen.

Expected Results:  
The default panel should stay on the laptop screen because it is the primary output.

This worked as expected in plasma 5.4.3 and changed after I updated to 5.5.3 this morning.