I rewrote or expanded a chapter. Why did its originality score disappear?

That's intentional. The originality score describes the exact text that was evaluated — once you rewrite, expand or otherwise edit the chapter, the old score no longer applies, so BookyAI clears it rather than show you a stale number. To get a fresh score, open Quality and click Check originality on that chapter's card. Chapters that still hold a score have a Re-check button in the same place, so you can re-run the evaluation any time with one click.