That was the moment I had my doubts. So I got another chair and tested it with it. No problem, screen stays on.
Took the Ikea "MARKUS" chair again, sat down - screen went off again.
HUH?! My chair has a feature that automatically turns screens on/off? I guess nice privacy feature?
I started to lose faith and couldn't find a logical explanation. So I searched the internet for "ikea markus chair screen problems" 🤦♂️
WAIT! I am not the only one with this problem!? Found the following (German) forum post:
https://www.computerbase.de/forum/threads/hauptmonitor-wird-sporadisch-kurz-schwarz.2060771/