I exported my book and got the old text, although the editor shows the new chapters. Why?

This usually means there are two books with the same title: the original and a copy made by "Regenerate all" with "In a copy of this book", and the export was made from the other one. From version 1.9.24, a copy's page says "A copy of" with the original's title as a link, and the original's page says "Has a copy:" with a link to the copy. When both have the same title, the line says so: export from the one you mean, or rename the copy by clicking its title on its page. In an older version, open each book with that title in the Library and check its chapters before exporting.