Horde spawned at the same time in that tiny location right in front of me and are stuck mostly behind the vines. As you can see, some of them spawned on the outside of the vines but could not move. It doesn’t look like a lot of guys in the picture but I found out they were all standing inside of each other when I decided to throw a bomb.
Festering ground map.
Edit: forgot to mention that while they were stuck, they were wiggling like crazy and it looked as though they were dancing.