ROMs don’t change the bootloader, this is dependent on what keys are used to sign the system image and verified boot metadata.
DivestOS uses unique keys for each component of every device.
I strongly suspect FP3 bootloader has the AOSP test-keys marked trusted.
Any ROM with unique keys is supposed to show the “different OS” message.
If you do not see a yellow screen on a locked verified boot device with an alternate system the bootloader should be considered broken.