It could be that for some ridiculous reason the portal simply won’t move to where it should. This was reported a while ago, and I thought I fixed it after then, but apparently not … I will change it into a static position with the next update, though.
For now, you might have to cheat if you really want to check it out, though you won’t miss out on that much, so feel free to skip it. When you access the area, open the in-game console and type “character.warpcursor true”, and false to turn it off again. The portal is outside the accessible area to your right when entering the Passage I, roughly between the entrance and the exit, and does have a map marker when getting close.