I had the same thing happen yesterday. I started the day in class D, but after a few races I was promoted to C with a 3.45 (started my final D class race at 3.98) and the MPR for C was apparently fulfilled. Give it a few minutes and refresh the page and you will find that you still need to get the MPR in

The upside is that I can just wait until the next promotion (29th) and I'm in B (still need to run the C class MPR though). I don't have anything that can run a C class race right now and I'm waiting for the next line up to see what I will buy.
It's down to the Ford GT, Star Mazda or the Indy car (road tracks). The F1 will have to wait a little