Situation: I left the instance to find a replacement for a character that left the group. When trying to re-enter the instance with my party (i never left the party) I enter the instance and find myself in a new one with everything reset and my teammates not there.
I think it's a bug.. Happened to my party once a couple of days ago when we tried to find a replacement before the final boss. It created a new instance and they could rejoin ours. Eventually we just gave up on it and quit. I have been recruited by groups that are already partway through the dungeon and I was able to join the instance no problem. The other time must have been a bug.