MS Word 2013 - Page number cross references are incorrect after updating

Hello,

In Word 2013, I have a largeish document (475 pages at the moment).  It will be used in print format in some cases, so I need to use cross references to point out page numbers of other sections (e.g., "For more information, please <section heading name> on page <page number> of this manual").

I create these via Insert -> Cross Reference.  The Reference Type is Heading.  The Reference To is Page Number.  (I also use references to Heading Text but those seem ok.)

It's been working brilliantly until today.  But suddenly when I update cross references (CTRL+A to select all, then press F9), the page number references are coming up seriously wrong.

I mentioned that the document has 475 pages at the moment.  But after I update cross references, a cross reference that should show page 468 shows page 603 instead.  There is no page 603.  There is no page 503.  There are currently 475 pages.

CTRL+clicking the link jumps me to the correct page (468 in this example).  It's the displayed page number that's wrong (not the link, apparently).

All cross references to page numbers are incorrect after I update.  As another example, early in the doc, a reference that should show page 18 says page 26 instead.

When I undo the cross reference update (CTRL+Z), it goes back to having outdated page numbers (but at least in the right ballpark).

I've pasted in some content from another Word doc today, but I've done hours of work since then - I'm loathe to go back to my most recent backup from before today.

Anyone know how I can fix this?  Please help!

Thanks!