Lost chapters in Pages document; how to restore?

I wouldn’t assume the chapters are gone yet. Seeing the same shortened document on your iPhone can simply mean iCloud synced that version to both devices. Collaborate on Pages documents and keep them up to date with iCloud – Apple Support

If the document is stored in iCloud Drive, check its version history before making more edits. On iPhone, in the Pages document browser, tap Select, select the document, then tap Versions. Preview the older versions and look for one that still has the missing chapters. You can Save a Copy instead of replacing the current version. Restore an earlier document version in Pages on iPhone - Apple Support

On a Mac, the equivalent is File > Revert To > Browse All Versions. Restore an earlier document version in Pages on Mac - Apple Support

I’m not aware of a normal Pages page limit that would simply remove later chapters. Version history is the first thing I’d check here.

On a locally supported drive, Pages will autosave document changes as you introduce them within ten seconds. This may be slightly longer with iCloud Drive as it is dependent upon your Internet connection. Document versions are updated on the opening, or manual saving of a document.

There would also be document version gaps where the Pages document was left open for a long period of time on a Mac that was not rebooted. That may sound bizarre but I have encountered users in the communities over the years that do just that.

Do you use Time Machine to back up your local Mac drive? That may be your only recourse to retrieve a separate Pages document from the Jan 2026 backup timeframe and incorporate that otherwise lost content. If that Pages document was stored and then removed from iCloud Drive, you would have had a 30-day sliding window timeframe for it to be restored on that medium.