Hello @Raibi I'm not really sure as I was never really affected. I've only heard my teammates posting images of this earlier and people mentioning it when we have played together, but since I didn't see any post topic mentioning this I decided to post one.
What I've seen and heard, all cases were larger than -1,000,000 exp lost, but that may not be true for all cases out there, simply what I've seen.
This is just because it's the CTE. The interactions between the CTE and your normal game character are a bit weird. It seems the CTE wants to reset everyone back to level 0, but doesn't do it reliably. Sometimes you'll get reset mid game, which shows as a massive XP loss. Playing the normal game and then going back to the CTE can cause it to happen again. I've also seen weird things happen with unlocked cosmetics.
But it's just the CTE. I've never had any permanent effects on my character when I've gone back to the normal game.
I get around -6,000,000 to -7,000,000exp every round I finish on CTE. It seems to be the higher the level, the more(or less) you receive. @Raibi Should us players stop using the CTE if this becomes a bigger issue? Also, the test server shouldn't even be communicating with the full game servers at all. Period.