September 2017 - last edited September 2017
On http://bf1stats.com/ you can see played and finished rounds, but i think leaving surely means a loss for you.
Edit: That was the case in BF4 if I remember correctly, there is no effect from quitting in BF1
https://www.reddit.com/r/battlefield_one/comments/6br6bg/absolutely_zero_punishment_for_quitting/
September 2017
Currently it seems leaving a match before it ends will negate the loss penalty if you do so before the end of round, but it'll also prevent the score bonus for finishing a round.
September 2017
September 2017
Clearly not all things are lose, I've noticed that my progress in medals are still counted if I leave a server. But I don't leave for any reason, even if we are losing bad. I'm referring to losing connection for some reason and getting kicked out of the server for such a reason. It shouldn't make much of a difference I would imagine.