I tried a contract with targets that were easy enough to kill the required ways. Upon finishing it I could hear a very strange looping sound like when you complete challenges, but it was a black screen with
(X) Skip
at the bottom. Definitely buggy.
Then I tried it with the regular mission. That’s only when the challenges were recognized. So maybe IO forgot to allow them to be completed via Contracts. I say “forgot”, but I’m sure there’s more nuance to the situation. I’m sure it wasn’t intentional or to force the players to do it a certain way.