This has happened to me twice that I am certain of.
Yesterday, I was checking the status of my Horde characters, in particular, Del. I reached level 15 with Del about 2 weeks ago, and 2 days ago Del was over halfway to level 16. I don’t know the XP level, but he was somewhere between 15.5 -15.75. When I checked his level yesterday, his level was less than 15.25.
This happened to me also with my Kait character. I lost a half of a level over the course of a couple days.
Anyone else seen this?