I made a service using something like a 64 bit wide ULID but there was never a presumption that data is be inserted or updated earlier than the most recent record.

If the domain is modeling something like external events (in my case), and that external timestamp is packed into your primary key, and you support receiving events out of chronological order, then it just follows that you might insert stuff ealrier than you latest record.

You're gonna have problems "backdating" if you mix up time of insertion with when the event you model actually ocurred. Like id you treat those as the same thing when they aren't.