to get the xp to stick you either dont go over 10k score or have a host migration sometime in the match. So you could leave or your friend (whoever is host) at the last 10 seconds of the match after you've gotten all the kills u wanted and then whoever wasn't host will leave once the host migration is over. Also if you are passing 10k score it usually doesnt stick all of the time unless you have a host migration. This works for me.