Carriers AI is broken(confirmed in the unit tester). Even if attacking units are in range of the carriers(not even just the interceptors leash, which should be what matters anyways) carriers will attack overlords sometimes rather than the attacking units which should have higher priority, this is normal carrier range, not leash or vision range. Test it out for yourself.
Come on blizzard. We know you want only Terran and Zerg to compete(judging by balance) but this is a complete joke. I am extremely surprised no pro has noticed this. I guess Protoss just doesn’t whine, we just try to win despite the balance/bug obstacles.