I don't know if this is a stock problem. Came down a two mile, 6% grade hill the other day and was treated to quadraphonic brake grinding. I couldn't imagine that all four brakes would start grinding at the same time. After the brakes cooled I felt them and there were no grooves or surface damage. I did feel tall rust ridges around the inner and outer circumference. I theorize that the heat from the braking made the discs expand slightly making the rust contact the pads. There has been no grinding since.

 

Not really common, but I can see things expanding when heated up like that. Ridges in and out are common (oem rotors are soft.) If the pads are all at serviceable levels, I'd let it ride.
Note that MINI suggests/requires rotors to be changed with pads.

 

and do not use the brakes on a downhill to control speed, downshift and use the engine

 

Once they cooled the grinding was gone. I don't drive it a lot because I ride my scooter as much as I can. So, it sits and the rust grows here in the wet northwest. I'll take the wheels off and take all that stuff off.