Killed outside Blood Altar zone, The Menagerie

I'm pretty sure this is not supposed to be possible. I finished the Blood Altar challenge and then there were a lot of these enemies with one blue one.

For some reason the enemies did not despawn once their challenge was over. I started another challenge and got body blocked and deleted nearly instantly. I went back, did it, and the blue enemy and a lot of other crab monsters were still there. They would follow me out of the arena.

I seemed to be desyncing or something sometimes when the blue one would hit me, and it felt like I was being hit like a truck by "nothing" during the fight. As I write this, I'm staring at the death screen while it walks around outside the arena.

I respawned and it's still there in the instance. No idea what caused this.