I've completed the first four missions of the game, achieving ratings of either Shadow, Specialist, or Silent Assassin on each. My scores are still there on the leaderboards, but the ratings have been reset to Agent, and the individual scores for each chapter of a mission are now 0.
This might have been only when I happened to notice, but I'm pretty sure this issue was caused when I went back to replay the scored section of the intro/tutorial mission on Purist difficulty (from level select, not the "new game" option on the main menu).
So what's up with this? Did it really reset my scores because I went to replay a mission, as if I were starting a new playthrough and would enjoy having all my records erased? Or is this is a bug? Either way, it's pretty annoying.
Some pics of what I'm talking about: