Kyrati Films racing/survival bug
This has been reported on numerous websites, is Ubisoft even aware of this bug? On your second and subsequent playtrough, all Kyrati Films missions you've finished in the previous game(s) are not reset, so even if you play them again for the first time, the game only counts it as you replaying a mission you already finished. This way you can't unlock the vehicle takedown and chain takedown skills.
The only fix to play any Kirati Films you never played in your last game, or if you're finished them all, you have to delete your save file and start anew.
This is a major bug, and this game has been out for while, I can't believe it's still not fixed.