My DOCX export is garbled from a certain chapter onward, but the PDF looks fine. What happened?

This was a real bug, fixed in v1.8.1 — and your book repairs itself. It affected manuscripts imported from Word: Word documents can contain invisible "break" characters that survived the import and made the exported DOCX file technically invalid from the first such character onward. Word would silently "repair" the file when opening it — and that repair is what mangled the later chapters. The earlier chapters and the PDF were never affected because they didn't hit those characters. After updating to v1.8.1, simply export to DOCX again: the export now cleans those characters automatically, and newly imported books are cleaned at import. You do not need to re-import anything.