Weird combination of events I suppose but here goes (TL;DR below):
I was on the scavenger hunt, first stage, but had only the slime statue to go.
I queued up for a random dungeon in the hopes it would be sewers.
It took a bit so I figured: ‘I might make it past the mobs on my won as a paladin’ and entered the Sewer dungeon.
The queue popped and I accepted, it was Sewers, so I got ported from my solo Sewers to the queue Sewers (was wondering if it was just 25% chance, or if it would be related with me being in already).
We completed the dungeon. Was wondering where I would go when I took the portal out (and return to where I was before), turned out to be Sewers. No problem, was done there, so took the portal out again, again ended up in Sewers. Did this a few more times, same thing, not even a load screen, still Sewers.
Logged out, logged back in was now able to exit Sewers. Clothes still smell, but that can’t be helped.
TL;DR:
If you are in a dungeon while you are also in a dungeon queue, you can get stuck in the dungeon if you accept the queue and it turns out to be the dungeon you were already in. Logging in and out will free you from the dungeon loop.