I chose third person but my fiction book, or parts of it, came out in first person. Why?

This was a bug, fixed in version 1.9.4. Choosing "Third person" in the wizard sent no narration instruction to the AI at all, and fiction with no narration choice behaved the same, so a memoir-like topic or a story bible written as "I" could pull the whole book into first person. Since 1.9.4 third person is stated explicitly for the outline and for every chapter (close third person limited to the viewpoint character when you set one; "I" allowed only inside dialogue). Existing chapters do not change on their own: regenerate the affected chapters, or use the editor's AI rewrite to convert them.