Photographs Disappearing from Older Albums in Photos

I've been composing albums in Photos since 2009, first with Apple's proprietary album-making software, and then when Apple discontinued supporting it, with Mimeo. Here's the problem: The albums composed with Apple's proprietary software are riddled with missing photos. The originals still reside in my library, but the albums no longer link to the originals, so they don't show up in the albums.

I can't think of a possible fix, other than rebuilding each of the albums. Any clues? I still have the PDF files that Albums generated, so all is not lost, but those can't be edited of course.