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 rpi-...