Update: it turns out that flashing buster instead of bullseye did the trick.
Would be interested to know if I did something wrong...

Hello,

even though is thread is from a long time ago, I'm running into a similar issue...

Apperently, my RevPi connect won't boot from the internal eMMC.
Trigger was updating to bullseye...
- tried first from Windows
- later also via linux with rpiboot dd
- later also via linux with rpiboot and ...