@Darknessoft Hi there, I hope I can clear this up.
You're not actually losing levels or progress, what's happening here is that your Match Summary screen is bugged and is telling you that you earned more progress than you actually did.
In your case the match summary is working on the incorrect assertion that you were at 8/10 stars towards your next BP level, regardless of how many stars you actually had.
You can test this yourself between matches by checking how many stars you have on the BP/Quest tracker page, then manually add the stars you earn in a match. You'll see that progress is being correctly tracked. You'll also see that you're getting a lot more levelup notifications than actual levelups.
This is being looked into and is an understandable source of confusion, so I hope this helps clear things up.