Express Scripts review: Mysteriously stopped covering generics

I used to take the generic for Farxiga. When I was forced to return to Express Scripts suddenly they said they would not cover any generic version and would have to switch back to the name brand at $500 a month. It used to be that insurance plans pushed you into taking available generics but now I suddenly am being forced to take the name brand. I can only hazard a guess as to why this is happening but I fear expressing it would result in some sort of SLAPP suit.