Torghast - Mort'regar floor 5 went back to floor 5

As the title says we killed boss of floor 5 and went into portal only to be back on floor 5 instantly dieing, as we ran forward thinking we where in floor 6s safe zone. this came right after 3rd member DCed and came back but on a different floor 5 then us. they then left party we continued to kill said boss.