I haven't encountered this bug, but whenever I encounter a sunken treasure chest in a dungeon-- such as in Blessed Crucible in the room just before the final boss-- I move well away from the area, reloadui, and when I go back to the area it's fixed. Don't know if that would work here, but it's worth a shot.
I also dropped the quest, shut down the computer, the following day I returned to the dungeon, redid the quest but it was still bugged. I reloaded the UI, swapped characters, left, went back in, dropped and re-did the quest several more times, did it with another character successfully and sacrificed a chicken in my back garden.
Finally, the portal was where it was supposed to be and I completed the quest.
'Sharp Arrows'Mr.Garlic
Hidden by darkness, a shadow in the night,A sped arrow dissecting the gloom,Finding it's target, such delight.
In 2026 this quest is still bugged. Impressive. And we're still forced to repete it until it works, exactly in the public dungeon no one wants to repeat anything.