The horror’s have a ranged attack that can paralyze you for a short time. I’m not certain but it may be possible for multiple (or maybe even one if yer really unlucky) to chain stun you. And since the game doesn’t tell you what happened until your next turn it never told you that’s what happened. They used their paralyze every chance they got, you failed your saves particularly hard, and they beat you to death while their paralyze was on cooldown.
As for not all the Horror’s showing up, yea, that seems to be a problem. I have yet to get anything from an Amigara fault.