Deterministic PRNG and the Firemace spawn?

We know that the Doom engine's pseudorandom number generator is deterministic and relies on entropy of the player's input to be "random".

We also know that the Firemace in Heretic is set up to randomly spawn at one and only one of the spots set on a map, and that there's a chance it won't spawn at all.

Given those two things, a fresh warp into a map (with nothing to have disturbed the PRNG's state) should always have a predictable (and therefore manipulable) result with Firemace spawning too, right? Or is there something I misunderstand with these mechanics? At what point does the spawn become (for practical purposes) unpredictable? After a demo runs? When restarting after a death?