I completed a contract but missed some bonus points, so I did the contract again to get them all. Each time I turned in the contract after getting the bonus points, the number of stars I have didn't change. It showed me that it gave me a star, but my stars were unchanged (3).
Is there a limit to how many stars you can have at once, or this just bugged?
Edit: here are some screenshots before and after, except this time it was doing the contract for the first time:
It seems that it caps it at 3 stars, and throws away any stars over that amount. After I took the screenshots above, I bought the next sniper contract and then restarted TF2: