I dunno how many times I've had the game just straight up cheat on me. Just had it happen again. The problem seems to be firing lines. Enemies can sometimes fire through walls, through closed doors, through FLOORS. Just happened in the Zenthium section of the goblin camp (They were holed up in their smokepowder room, door was closed, they could shoot out but I couldn't shoot in.) Other common places are the ruins just outside the Goblin camp (where Volo is) and in the spider nest fight with the queen phase spider (that's the most common one for shooting through floors.) And it is NEVER reciprical . . . they will be able to shoot at me all the ding-dong day but I can't shoot back at them. Doesn't matter if I have the high ground or they do.
