The problem is a lack of troops. Also, why are you going to only one crypt when there are four? Replay until you get the desired outcome. Here, everything depends on random chance.
There is no randomness there, and the AI always behaves the same way.
Ihtiandr has exactly 42 gnolls, 20 lizards, and 1 fly in his video, and he clears exactly the same crypt.
I specifically did a test, and everything worked normally for me. Will you send your save file for testing?