Did not see this one in the list of known issues yet, so:
On multiple occasions when I joined a game that was already in progress, the post match summary says I got 0 xp for the game. I'm not sure whether I got any xp or not.
In the screenshot below, I joined just after the match had started and I got MVP, though according to the match summary I earned 0xp.
PS: Enjoying the beta alot. Game is really solid. Wish you the best of luck with the optimization.