Depending on when the guy went AFK, I do think it can be unfair.
I've lost a few games from people that disconnect in the first 5 rounds, and I lost around 20 RR cuz we got stomped in a 4v5.
If I'm not mistaken I do think they plan on doing something about that, but they don't want to run into an issue where premades abuse this system in order to help one of their friends rank up faster.
So it's a pretty tough thing to solve imo