I ran into this issue over the weekend. I was helping a friend get mythic kara completed for two artifact knowledge, so I cleared the instance on m0 for him.
The following morning (prior to daily reset), I attempted to host my +33 lower kara key without realizing m0 dungeons can’t be reset. I invited a few players and we spawned into the library room where the shrunken fight with the mana wyrm normally takes place.
There isn’t an NPC to teleport you out, either. I think you just need to hearth. I totally screwed those guys over without realizing that was a possibility.
I had to join another pug just so I could reroll my key away from Kara.