The game is full of various bugs. This is just one of them.
I had to play 6 priests games to count as 3 for the quest. Funny enough, the 3 games I fully played didn’t count. Annoyed to the max about it, I just AFKed without even moving my mouse, and THOSE 3 games did count, and finished the quest.
Guess Blizzard don’t want us to play the game?
But hey! Buy mercenaries bundles!