How do I stop the AI from revealing a secret too early in my book?

There are two ways, and they work well together. First, in the Codex, under Relations, add the secret in "Secrets: who knows what" and set "The reader is told in chapter" to the chapter of the reveal: before it, the narration only hints. Second, from version 1.9.23, you can give an instruction for every chapter, such as "Do not reveal that Tom is John's son before chapter 3", in the "Regenerate all" window or in Book details, "Instructions for every chapter". If the secret already appears in early chapters, regenerate them afterwards, and check that it is not also written in the outline or in a Codex note, where the AI would read it.