I encountered this bug quite often, when I used random dungeon finder with some people.
Used random dungeon finder and got The Stonecore (if you go outside of the dungeon, the location is on the same spot as the entry, so you go out and instantly get back) Only happened in random dungeon finder so far.
Also when someone leaves and you get someone random, go back to the dungeon, the dungeon somehow resets for everyone and all bosses and npcs are available again.
I noticed the same result in Deadmines when I used v13.1 Gold using Solocraft's Random Dungeon Finder and died.
Played Lv19 Alliance Gnome Warlock, entered Deadmines solo, got grabbed by second boss ( Helix Gearbreaker) which I couldnt escape and died.
Re-entering the instance, the bosses and mobs were reset.