1. This happened to me also, it isn't game breaking and I don't really know what caused it. I suspect it's because I chose to do the demon's quest on Bloodmyst Isle and then I killed him. But I can't be sure.
2. Try to use the amulet and refill it, maybe it will cause the game to update then.
3. Use spectral sight at the location of the portal. That's how it reveals.
4. Donno about this one, but if you have an older quicksave, maybe you can try loading it.
