Start off with a folder of freshly downloaded TMStadium tracks. Dbl-click the first, loads fine. Race, enjoy, hope to finish; then dbl-click the second, but the first track loads again. After a few more tries, the intended track load to MP without a problem. As I work my way through the tracks, the same occurs every single time.
Making it a lot more time consuming than necessary to test out all these new tracks.. hope it can be fixed ASAP.

Edit: FWIW, on Win8 x64 RTM -- though, doesn't make any difference.