Don't know if it already happened to anybody since I can't find here or on youtube anything like it, but the boss, just after my friend died to it, very, and I mean very, close to the exit of the dungeon, decided to ignore me completely and simply appeared outside, moving as usual and everything; my friend spotted it when coming back from portal.

Now I do have my hypotheses, but the reason should be besides what I already said; the boss executed its lunge or teleport from the ground (don't remember exactly, didn't payed enough attention) just after my friend died.

The fight resumed within a much better, and of course easier, battlefield, just outside the entrance, and next to a NPC tower and some terrain slopes, which the boss got stuck most ot the time, until we're done with it. The NPCs don't attack the boss at all, but I'm not sure if it tried to specifically kill them at any time.

Some things worth mentioning:
We don't use any mods;
We didn't do anything with the terrain next or at the entrance of the dungeon;
We are playing on a dedicated server, built by this friend, and we're the only ones at the server at the time, both (until he died) at the boss fight.

https://steamcommunity.com/sharedfiles/filedetails/?id=2921560476

https://steamcommunity.com/sharedfiles/filedetails/?id=2921560505

https://steamcommunity.com/sharedfiles/filedetails/?id=2921560523

My friend recorded most of it, when it got outside, and maybe we will upload it soon.

Edit: Now I see that the last patch where they nerf some stuff, they commented about fixing something which could cause the boss to escape, so there's that, it still can happen.