The DietPi image has only a single ext4 partition, hence no uEnv.txt is loaded and the distro boot target, which requires the 3rd partition to be FAT with extlinux on it, fails. The bootloader then goes on with the mmc0 target which scans all partition for boot scripts, extlinux, EFI in a generic way and supports all filesystems.
So if all other images fail, then the distro boot target fails or hangs at some point where it does not fall back to the next boot target anyhow.
However, serial console output needed to debug it further.