ARAM champion mastery biased against tanks

Hey, I just returned to playing ARAM after a long break. The last time I was playing was before champion mastery was available for ARAM. From what I can see, I think that the champion mastery grades are biased against tanks. Tank is a super important role in ARAM, but it seems like the system is only valuing people who go full ham. I've had some really strong tank games (racking up tons of assists, absorbing lots of damage, providing lots of cc) for which the system has only given me B grades. When I have those sorts of games on Summoner's Rift in Classic, the system recognizes and rewards them with S games. I think that ARAM would be healthier and better if the rewards system did a better job or recognizing the tank role. In any case, thanks to Riot for adding the champion mastery to ARAM--I think it just needs to be tweaked a little.