Yesterday I tried to change my strategy from "M, M, M, M" to "S, S, SS" but when I exited the strategy page and then returned back the game didn't keep my strategy changes. It remained "M, M, M, M". This bug occured with both App and PC.
Later, when I checked the race results, I noticed that tyres in race were "S, S, SS" like I set up.
If you are racing in 2 car league try to change laps per stint. If strategy which didn't saved is S(10) S(10) SS(5) change to S(11) S(9) SS(5). Similar thing happened to me and it was helped me.