The preview looks different after reopening the project.

When you reopen a book, the tool reloads your saved settings and re-renders the preview, which can look subtly different from a live editing session (fonts finish loading, the 3D view re-lights). That's cosmetic and the export is unaffected.

If something substantive is missing — your manuscript is gone from the Interior Formatter, for instance — the likely cause is that the project was created or exported before the manuscript was saved into it, or from a different browser. Re-load your manuscript. To keep everything together, work in one browser and download a project file that includes the manuscript.

If the whole book is missing, see My project disappeared.