This has been going on for months, somehow has been allowed to just become part of the game. But anyway, here goes:
When you queue into a dungeon with a premade (or possibly queue-assembled, I don't know) group, it often will kick one or more members out of the group and make them appear "offline" as the rest of the group ports into the dungeon. Those that are not transported remain wherever they are, and have to manually port to the dungeon to rejoin the group if they have not actually crashed.
There are workarounds but it would be good know the developers are aware of this and doing something about it.