Every time I start it, the settings change and it goes into windowed mode.

Hey Taka12300912​ Try deleting the existing Personal settings 1 profile after launching the game, or if that's not possible, make a copy and delete the existing settings folder from this location: C:\Users[Username]\AppData\Local\EA SPORTS FC 26 (AppData is usually a hidden folder, so it can be made visible from the View dropdown.)