Just had this happen, what I did to fix it was queue a follower dungeon, leave the instance, and then log out the character and back in. Now you are at sinfall, the quest is completed, and can move.
As of 25 September 2024, you can log out to your character selection screen and log back into the character again. At this point, you can move to go interact with the scroll which brings you to the upper level and go through the mirror that you used to get to Maldraxxus to return the quest!
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Crazy that after all this time this is still an issue. Your method worked very well. Thank you for that, as this just happened to another character I just played this on.
Still bugged, luckily there are some uploads on YouTube if you still want to see the after battle cutscene.