It’s a visual bug. The weekly challenges look like they give 70k XP instead of 35k, sometimes looking like you levelled up when you haven’t. When you go back to the lobby, this “ghost XP” disappears, making it seem like you’ve gone down a level, when in reality you never went up a level in the first place. Yes I agree that this is a bug and that Epic needs to fix it, I’ve just been seeing loads of these posts lately.
Thanks for explaining this! We're tracking this XP Bar visual issue on our trello page here /u/MaxiorTM, and will update it with our latest info.