I've noticed this with a few of my players recently, particularly with ones who are having a bad season and play in DMC/MC. They seem to score and yet still get a 6 rating afterwards. I had this a few seasons back with another MC who kept scoring and generally making a big contribution in matches but still ended up getting 6 ratings. Eventually his form turned (started getting 7/8 ratings) and he became arguably my best MF.
Is it best just to persevere with these players when the game is maybe giving them troll ratings like this?