Lightroom Classic: book module doesn't clear previous images

Does it still happen if you:

1. Close and Reopen Lightroom?
2. Restart your computer?
3. Reset your preferences file?

Close and reopen LR - yes, absolutely
Restart computer - yes, absolutely
Which preferences file do I reset?  Did I set preferences for the book module somewhere that would relate to this issue?

I just saved a book using only used photos.  The extraneous photos are now gone from this saved book collection.  When I select another collection, there they all are with new ones from the latest book.

A Preference File will survive a Lightroom uninstall/reinstall. Sometimes weird behavior is corrected/cured by resetting the preferences. 
Reset Procedure:
1. Close Lightroom.
2. Hold down [Alt/Opt]+[Shift] while restarting Lightroom. 
3. Overwrite the Preferences when prompted by the dialog.
4. Close Lightroom.
5. Restart Lightroom.
Does the behavior continue after resetting the preferences?

Yes, the behavior continues.  The filmstrip in book module continues to accumulate photos from previous books.  It has now added photos from the book I saved earlier today. 

When I saved the book earlier today the Book collection included all of those extraneous photos from the filmstrip that aren't in the book!

I still get this issue. (Windows 10, Lightroom Classic 10.0.)

Is there an explanation and/or solution?