Guessed/reported - past simple/perfect

I have the following two sentences:

The teacher guessed that some of the children had cheated/cheated in the exam.

At the conference, the scientists reported that they had found/found a cure for malaria.

(English Grammar in Use - Advanced - Cambridge)

The key in the book says that in the first sentence both work, had cheated and cheated, while in the second sentence only the past perfect works, had found.

I don't understand the difference between the two. I'd have opted for past perfect in both sentence, but why should it be fine in the first sentence and not in the second sentence?

Thanks!