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I have been paining over this since the update to 16.2.1. Screenshots below show a quick sample book I created to see if I could get it sorted.
I gave up and just reverted to 16.1. Here you can see the page numbers working as they should.
I can see no issue if I create a book file like yours with InDesign 16.2.1 on my Windows 10 machine.
All documents were created from scratch. Facing pages documents, 5 pages each. Just added to the book file.
You could download all my test files from my Dropbox account to see why something is going wrong with your book file: https://www.dropbox.com/s/phdmldsx49zfvrf/InDesign2021version220.127.116.11-Book.zip?dl=1
( ACP )