Me and a friend are both having this issue, and I have seen players in global chat complaining about inconsistent payout in ZM. If you start another round of ZM after playing once, all games of it afterwards will not award units. You have to restart your game to make ZM payout again.
So far ZM is the only gamemode or form of payout I have found that has this consistent behavior.
I posted this in the Steam Bug report forum but was told in global chat to post it here. Sorry for redundancy.