EDIT: Since the other topic is so long, lets keep discussion here.
All images are DietPi v8.14. Both images (Bulllseye and Bookworm) below the testing sub directory are shipped with Armbian’s U-Boot (EDIT: I just see I didn’t built Bookworm with it), the two ones in the parent directory are with the U-Boot build which worked for you before, and has not changed. There is neither a difference between Bullseye and Bookworm, nor between DietPi v8.14 or other versions in this regards.
Can you show the log messages before initramfs prompt? There was no other drive or card attached at that time? In my case, with eMMC and SPI switch, it didn’t even reach initramfs or kernel stage: after the autoboot timeout of petitboot ended, it just did nothing. That all shows me that booting via petitboot seems highly unreliable, at least with mainline kernel.