UPDATE: In the Pro events, I noticed the Fully Upgraded events were unlocking, so I went to Indy, did 10 laps in a different muscle car (the Firebird) and then went back to Semi-Pro and noticed that the muscle event was unlocked.
I did another 3 or 4 series and it unlocked each time (Pro-Muscle and both Tuner events in Pro). So the issue seems limited to the 1967 Mustang Fastback so far.
Having the same issue, what you mean you went to INDY?
This. Same issue and for some reason doing what you did fixed it! Quick custom race, 10 laps around Indianapolis oval in the firebird, and it unlocked! Very strange bug though, still bugged as of April 23.
