On Festering Ground, there is a spot where the enemy AI from specials, horde enemies, elites, and even Monsters get stuck on a set of vines unable to proceed.
I have had this issue multiple times on festering ground even to the point where Chaos Spawns have gotten stuck on the vines unable to attack the players and get downed without getting past the final set of vines.
The location is very shortly after the second grimoire location hitting up the wooden ramp and through a narrow valley that leads to the 3rd Tome on the map. The enemy unit will down vines on the way to the players, but just gets stuck on the very last set.
Its very easy to reproduce as all I need do is stand around waiting for specials or horde enemies to come and enemies have gotten stuck here without fail that I have seen. Happens every run unless players destroy the vines that I have seen.