My guess is new firmware started doing the same thing that Kindle for Android is doing for show/hide of Sync button: the app keeps a database with list od ASINs associated with your Amazon account. IF book/ASIN is in your Cloud archive >>> show/enable the button, else hide/disable.
That's why valid ASIN is not enough.
You could check that theory by editing the ASIN for "problematic book" to match ASIN of any book that you bought from Amazon (or sent to Cloud archive) and re-send it to your Kindle.
My PW3 is still on fw 5.6.5 so I can't test that. I've tested the Android thing months ago...
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