I found out what was the problem:
While I was testing I used the power button to turn the system off gracefully. It turns out that Rock Pi X does remember this: If the system was turned off via the power button, it will not turn on automatically if the power gets cut and later restored.
However if the power is just cut and restored the Rock Pi X will boot automatically.
I was expecting that the BIOS would contain a setting which would adjust this behaviour, but at least the current version doesn’t.
Also I got a fast reply from the support email from Tom, thanks for helping me out!