My paladin crusader is supposed to be fully immune to poison, yet keeps getting put to sleep by the spider venom of the spiders in the Chapter 10 Crusader cave area. I thought he had fallen under a spell, but there were no casters in the area (the corrupted nymph had been killed at the beginning of the battle), and no indication of a sleep spell/ability being used in the chat box. However, after being attacked and hit by one of the gargantuan spiders, my paladin crusader fell asleep twice.

I believe this is a bug, but wanted to see if someone else had encountered this issue. I might be completely wrong about this, but if so I was hoping to get an alternative explanation. Thanks in advance!

I have included a screenshot showing that my paladin (Robert Hillendale) had to make a saving throw immediately after being hit. I believe the class should be immune to this, but wasn't sure if that was the case.