I am playing on PC (Steam), Definitive edition. No mods installed, and I am not using any of Larian's gift box mods.
Here's a video clip of the bug:
https://www.youtube.com/watch?v=lMwFQCR4baMEssentially I'm demonstrating the fact that I have no mods installed, and that I am getting two times the attributes. Removing polymorph only takes away one, so effectively the extra point is permanent. It also does not matter whether or not you spend the points inside, or outside the mirror. (See 1:12 and 1:39 for comparison)
Also I didn't record this, but it also does not matter whether or not you spend the points/keep it in the pool. Doing this just gives you more and more points.
The kicker is that this can be done an arbitrary number of times, by taking away Polymorph points down to zero, exiting the mirror and doing it again (I also show this in the video). Unfortunately, I don't remember how many points my characters are
supposed to have, because I have done this a few times before I noticed it was happening.