Hi @TheYoctoJester ,
thank you for this speedy feedback. Great forum with lots of tips here! Misused the word “fork” on my end, apologies 
What I tried so far with my CM3 device:
Flashed the original RpiOS (Bullseye, 32-bit, latest version) to USB → Booted fine
Flashed the mender Rpi3 image from here: Prepare a Raspberry Pi device | Mender documentation to USB → Did not boot
Run mender-convert on the original RpiOS from above, flashed to USB → Did not boot
And while writing this post, I decided to stick the USB in my normal rpi3 (not cm3) to try if it works and it looks like it does not, while it does work from sd-card. I just never noticed because I always run standard pis with sdcards and my CM3/CM4 devices have either USB or eMMC boot.
Which leads me to believe my issue is a duplicate of this thread?: Booting a converted image from USB - #5 by sdynes
I’ve also got some CM4 / CM4S devices (can only test with them next week) which have an integrated eMMC storage. Is eMMC also affected by this issue?
I can gladly test the workaround “kacf” mentioned in the other thread, but I do not know how 
Kind regards