A subtle bug that had been slowly degrading a subset of my textures up until now. In the video, I made the editor reload the texture after every save and saved changes repeatedly. The strange pixel deterioration is caused by the premultiplication/demultiplication phase. You see, the color channels are stored as 0-255 range integers, so I must convert them to float before applying/unapplying the alpha. I forgot to include a round() to the final result, so the RGB values were slowly getting floored away.