I can confirm that this is still happening. I was grouped with a friend to run through the new companion missions. In both portal instances, I finished the dialog slightly ahead of her, and my version of Zerith-var opened the portal, and I even had the prompt to use it. Then my friend finished the dialog, and her version of Zerith-var walked over and essentially cancelled out the existing portal. She said that on her end, she could see my portal as he was walking over to it, and then it disappeared. We did not try disbanding; we just manually traveled to the points in Khenarthi's Roost and Reaper's March where the portal would have taken us (which appeared when we chose "show on map" from the quest after the portals disappeared).

I can also confirm that relogging does not help, because we stopped for the day when the first portal disappeared, and the portal was not there when we picked it up the following day (still grouped).

For an MMO, it's amazing how many quests do not appear to be play tested as multi-player internally and go live with bugs that only appear when grouped.