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It's much harder to kill the boss when she's stuck up there.
If you do kill the boss while she's up there, you can't loot the boss.
This has happened before. A few months ago when they had the bug with bosses not jumping down (most notably in Wayrest Sewers), the Mage would always be stuck up there. Now that bug has been fixed, I haven't seen her stuck up there since. Must be pretty rare.
Anyway, it's a bug. The reason why isn't not an exploit is that this bug conveys absolutely zero benefit to the player. It makes the fight harder and screws you out of the final boss loot. If this happened to me, I'd be very annoyed because it means that we'd have to restart the run and all that time was wasted.