It makes sense for each component to check signatures of its code to prevent various kinds of attacks -- e.g. someone coming and reflashing just infotainment or motor controllers with something malicious.
So, OTA update comes in, containing a bundle of software for different subsystems. It's sent to different subsystems. Then, those subsystems check integrity at startup, but one subsystem's bootloader isn't happy because the firmware looks to be invalid.
You can only prevent this if the OTA knows how to do equivalent verification for every subsystem in the car that checks integrity. (And, of course, even if you do this, there's other ways you can go wrong that aren't specific to integrity checks).
