Two weeks ago I had the chance to talk to one of ZOS members in a twitch chat were the host did white gold tower on vet difficulty on PS4 (I guess) and as one of the pulls revealed a bug of the planar not attacking the pin-clicker but one of the other members, the discussion turned into a broader discussion on further issues like the pet-taunt issue at Planar (no-one needed to click the pin), Molag Kena and her lightning wall ability, which could be avoided on using synergies and one of the nightblades ultimates, and also on a couple of issues at the final boss in ICP.
In general, there should only ever be 2 portals up right before the ultimate (in case no-one was dead or missed one of the two spots in a portal). However, there is still a chance that either only one portal is up (without any member using one spot) or even 3 portals up at the same time (without any left-over portals from a previous iteration)! I was therefore asked to post a screenshot when we notice this issue again. So here is a screenshot from one of our attempts tonight (yeah, ran with two new members and one of our usual DPSers went for heal so had to adept a bit therefore) where right after the start and before the first ultimate there were 3 portals up at the same time:
No-one was dead before, so no left-over portals as this also was the first ultimate in this attempt by the boss as already mentioned. This, especially with the feedback-mechanic on hardmode, can be an unavoidable death-situation which therefore needs to be fixed. If you can trace some kind of combat- or event-log on your side please have a look at the European servers. The time format in the picture is CET. The angle isn't the best, but I had to hurry a bit to reach the portal in time and also press the screenshot button in the same time.
Please also have a look at the barrage (channeld cast of the boss) where sometimes blocking this ability as a tank works like a charm (no-one except the tank gets hit by it) and the next time these bolts are just rushing through the tank (or slightly right or left) and ignoring the tank in front completely.
Also, the number of orbs seem to be limited to the health of the boss in some way - at least on non-hardmode. At arround 45% our healer had to pause for an RL issue and our two DPSers died therefore (barrage ignored me although infront of the DPSers), I was further tanking like 15 minutes without any new orbs appearing (we had like 7 up at this time, and the boss did his animation regularly). We continued the fight when our healer returned and ressed the others (still can't ress others while the first one did not accept the ress ...)
Hope this helps to spot the issue with only one or even three portals up while the ultimate phase.