Ye the reason is because ambush never despawns he just usually goes far away in this modifier but crucifix forces him to Despawn which breaks the modifier
There has been a bug exist for a long time where ambush will spawn in the upcoming room instead of the previous rooms. This bug has since been patched but can still occur if the player travels the rooms while ambush is at the back. There is a bug in “again & again & again” modifier where 2 ambushes may spawn at the same time and the player keeps travelling to the rooms while they are in the back, one of the ambushes or both may come ahead and one from the back. Making the player hard to survive.