Determinism isn't required unless it's required.

If it's not required, then you must plan for it NOT being deterministic, with any accidental determinism being ignored (to be safe, forcefully so with an intentional randomization/delays within the library). If it is required, then my random input should always (from the tests perspective) come out the same as I put it in.

If possible, force the corner case if the corner case is a concern. That's the purpose of testing. If there's a concern with timing, force bad timing with random delays. The alternative is relying on luck. I try to make my code as unlucky as possible, during development/testing.