Sorry for the late reply mate and another apology for not really being able to help you.
This should absolutely not be happening. There is also no indication in the script that anything is wrong and no one else has reported this before. Nazgul do however spawn the turn after they have been destroyed, so that part is normal.
Things like this occasionaly happen. Helm's Deep has a similar problem, where a reinforcement army will spawn on the hill above the Hornburg without a way to come down, making the army effectively useless.
Unfortunately, there is no real way to fix this. I myself don't have much knowledge of these things and King Kong and even Leo apparently haven't managed to improve it... So there is probably not much that can be done about it