If they know what is/was wrong, they can model what happens if the problem reoccurs even if they don't know why it occurred, disappeared, or might reoccur. But

If they know what is/was wrong, they can model what happens if the problem reoccurs even if they don't know why it occurred, disappeared, or might reoccur. But yeah, about the only thing more irritating than having a problem occur for reasons you don't understand is having it disappear for reasons you also don't understand.