No i am not using party bot, but dungeonbuddy with the partybot that is in DB to run in party mode, i use 1 tank and 1 dps, the tank will never move in nexus, same thing just happened with the tank in UP (Lvl 70).
At loading it does this but if i move it(tank/leader) to attack a target it will finally move and continue the dungeon. others i seen have said the same thing on loading the tank/leader that it will never move which makes the followers never move. It seems some dungeons the leader if its a tank will not move after loading into the instance.