Not sure if its a bug or something else, but I've noticed the amount of sponsorship income to be gradually decreasing over the course of a season.
I've sometimes take the +marketing training to raise the sponsorship coming in - could that be affecting it? Could it be deducting 10% rather than adding it on? It's not happening all the time, and I dont think I've chosen the marketing training the last couple of races and the sponsorship income has remained the same, but if I think back, then it could be related to that.