When players join a game in progress they should not receive a loss on their personal stats should their team lose the game. If their team manages to win they should still receive a win on their personal stats.
This is the intended functionality so there is always an incentive for players to finish any matches in progress they join, we are of the issue where losing a back filled game will count as a loss in personal stats.
We are investigating the issue and hope to have a resolution in place soon.