It looks like mobs stop to react to players at all once the Line of Sight got interrupted becouse of the pathing (if the way the mob would have to take becouse of the textures would require the mobs to turn around) This causes a infinite loop of Line of Sight checks wich can't be interrupted anymore causing the mobs to dont fight or move at all once they get damaged. This bug occur mostly if you run uphill and crossing the summit.