It appears fixed in space relative to the dungeon - I can look at it from different angles, etc. It seems like maybe it's only there at night, leading to some speculation that it might be a weird glitchy copy of the light from Mount Satori, but I'd really like to know for sure before I beat the boss, given that afterward I won't be able to get back in.
Wherever this light is coming from, it is a glitch, and has nothing to do with the dungeon at all.
As for what it might be, the animation is far too fast for it to be the light from Satori Mountain, and comparing it to the updrafts, it has a similar speed to that (although the more magical appearance obviously shows that this isn't an updraft).
The game is known to experience a variance of graphical glitches, across different platforms, so it's not entirely unexpected, but at the very least it's not game-breaking.