Rex changed it so you have to flip all the levers to get the portal to come on. That means have to clear all the rooms, not sure if just the ones on the bottom floor with the portal are good enough or if you have to flip all of them in the whole instance. I normally do all and the portal opens when I do that.
There is a bug/feature that the quest will trigger complete when walking to the door that leads to the portal. Ran a couple times and seems that it only triggers once and just the players in range get the quest complete.
Your friend was probably the first one to hit that area and you and the other person weren’t in range.
Go in again and make sure you approach that door as a group and you should be good
The levers that you trigger in certain missions to complete objectives seems to work every time even with multiple people, could the sensor be switched to a lever instead at the gate?