Well today I tried to make some observations again, especially in my own team.
Player A: ST 1, played 86 minutes, remained at 8 GS (gained some DMI)
Player B: ST 8, played 68 minutes, remained at 7 GS (lost some DMI)
Player C: ST 6, played 80 minutes, dropped to 7 from 8
Player D: ST 3, played 73 minutes, popped to 9 from 8
Seems completely random...
I think there is just a mess up with the changes regarding ST effects in GS. Actually I'm pretty sure for that. I'm playing lots of season, it's the first time I witness this randomness.
But yeah, a better rotation system could help a lot, as long as the GS ups and downs are more strict and not based on luck. You don't need solid foundation for a "lucky" system.
Currently GS changes are bugged.