I've noticed a tendency for the ratings to be harder on players of higher stars. I've had lots of players that were getting 8,9,10 while 5* near negotiation level only to see them struggle to get 8s after promoting to 6*. This effect might be amplified on Gks. I've also witnessed a boost/decline in ratings post match depending on results. For instance, I've seen my Gk boost from an ingame 8.1 at 89 minutes to 9.0 post match in a 1-0 victory.
here's some evidence of mine.
Keeper faced 0 shots on target in an 80-20 match and still rated 8 under the old system. I think a clean sheet is an automatic 7.5 and bonuses are given to the rating for saves and passes completed. How is is weighted and the formula I have no clue. But until you can find a match for each of the two keepers where the scoreline is the same and they face an equal amount of shots and make an equal amount of saves we will never truly know if lower rated players are given higher ratings for performance when facing higher rated players.