SUMMARY

I use laptop which is often connected to the external HiDPI screen (4K with scaling factor set to 150%). When external monitor is plugged, laptop is closed.
Sometimes when I use laptop without external screen and then plug it into external monitor, all my applications are closed and I got empty desktop, like I would just log in into it.
I don't know exactly when it happens and why it is like that. I can provide some additional informations if You will specify what You would need from me exactly.
I use KDE on Wayland (because scaling factor works generally better on Wayland).

STEPS TO REPRODUCE
1. Close laptop (it should be configured to not go to sleep)
2. Plug laptop to the external HiDPI screen
3. Unlock screen

OBSERVED RESULT
All applications which were opened on the desktop are closed.

EXPECTED RESULT
All apps should stay up and running as they were on the laptop's screen.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
Operating System: Arch Linux
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.10.83-1-lts (64-bit)
Graphics Platform: Wayland
Processors: 12 × Intel® Core™ i7-9850H CPU @ 2.60GHz
Memory: 62,6 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 630