Alas, I'm pretty sure it's an engine bug.

I don't think this is a bug at all.
This is a *
feature* which was available since oblivion, and that is:
When a random monsters has a chance to spawn none, if in the first visit it does not spawn any monster, it is not marked as used, and in the next visits, it has a chance to spawn monsters, until all the random monsters are marked as used, and then the cell has to reset to clear those lists from their *used* mark.
So if a cell has a lot of monsters that have a chance not to spawn, then the next visits of that cell might yet have a few surprises for us.