As far as I can tell from experience, it's a bug with the Minis. The wings can only be adjusted if you have run a session with cars that have adjustable aerodynamics before hopping into the Minis - if the Minis are the first cars you are driving when you start up the game, the aerodynamics are unadjustable like they are meant to be. I haven't tested it myself, but I'll take Ryan's word for it and say that it makes no difference anyway.
I thought I went mad (as I didn't see any Steam updates) when I encountered this myself. After running the Porsche 997 I was able to adjust wing + splitter + disconnect (0 N/mm) both anti-roll bars on the Mini.