I was just in a T2 graveyard with a terrible group. Shenanigans occurred just a couple rooms into the level, and I knew it was time to either quit or go solo. I went solo, thinking the map would change to an easy graveyard with one phantom. After loading, all four phantoms were still hanging around the area where they'd massacred my group. I killed them and moved on, heading past the entrance on my way to the next area.

There were three more phantoms at the entrance, plus a fourth which wandered in from the uncleared part of the map. I killed those too, and of course since I was spending all this time soloing phantoms I had to kill the four of them two more times before finishing the level. I'm assuming the original four had despawned completely and were replaced by the four I found later, because I frankly don't believe I'm badass enough to solo eight phantoms.

It looks like the extra phantoms don't despawn when you go solo. This is a terrible oversight.