It's happened at least three times for me, and not small amounts, either. I went from 40-something% EXP progress to 18% one time.
After it happened the first time, I kept a screenshot whenever I finished playing of my EXP bar's progress percentage, so the second time it happened I sent a support ticket.
They said they'd look into it, but haven't gotten back to me yet, or returned the EXP I lost (it's been a few days). It's since happened once more (from 40% to 13%).
I'm only on quest 2 of 5, and it started happening after I reached level 50 for the first quest.
If it hadn't happened, I'd have finished the second quest by now, probably even enough for another level on top of that.
I just wanted to let people know that it's glitchy, at least after reaching level 50, and while the amount of stamina potions I've saved up should hopefully make it possible for me to successfully complete quest 2, it's come down to having less than 24 hours to do so.
So beware, and if you do play, keep screenshots of your EXP bar (hovering over it with the mouse shows your current percentage) in case you need to send a support ticket!