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.
Let's see if it's fixed in V14.
I know a big update is coming
I've also had this issue.It seems that after dying in a dungeon finder dungeon and then reentering the dungeon it starts treating it as a new instance entered normally. I'm not sure how it behaves with multiple people if someone was there to hold the instance after a wipe.