I'm 90% certain the banshee's all-enemy put-them-to-sleep attack can only have one enemy asleep at a time. I've never gotten a second enemy asleep.
Also, not a bug, but I feel like having wandering enemies is kind of pointless when you get nothing for fighting them, and you need every resource you have to win the mandatory fights. I just reload the game if I run into one(or the traps in the ice maze) because I'd have to go back to the save point anyways, and it's faster than finishing the fight.