Relying on an explicitly documented implementation behavior that the specification explicitly describes as an option is not an issue. Especially if the behavior is only relied on in a test, where the worst outcome is a failed testcase that is easily fixed.

Even if the behavior went away, UUIDs unlike serials can always be safely generated directly by the application just as well as they can be generated by the database.

Going straight for that would arguably be the "better" path, and allows mocking PRNG to get sequential IDs.