I've had this problem since patch S came out, I'm currently using T. When I adjust a parameter in the setup, sometimes it adjusts the other setups also. Recently I changed the fuel in the BF1 to 18%, and even the FOX setups went to 18%. Has this happened to anyone else?

Doesn't the fuel always stay the same even when changing cars..?

The amount of fuel and the pit-strategy isn't part of the setup. Thus you can carry them from one car to another.
The wise man says: It's a feature, not a bug.

Vain

It can be confusing at first since the fuel setting is in with all the setup settings. It shouldn't happen to any other setting.