No? They still both have work to do regarding that quest, as well as didn't fail it. The quest was to solve it, not to subjugate it. I would argue that finding out the cause early on is just completing it early. That said, they still have a couple of nights of patrolling to do, as they need to verify that it was related to their quest, as well as verify that there is only one.
So they should report their findings to the guild, patrol a couple nights to ensure their suspicion is correct, and each day inbetween gather rumors and intel regarding whether anyone found the problem persisting.
So they should report their findings to the guild, patrol a couple nights to ensure their suspicion is correct, and each day inbetween gather rumors and intel regarding whether anyone found the problem persisting.