Hmmm... when my PW 10th gen installed 5.16.4, I did a reset/cleanup and it took a bleep of a lot longer to sync my Amazon books back to the PW that it took syncing 3 times the number of books in my Kobo account when I did the same on my Clara HD. The Clara HD also allowed me to hit download all after the initial 5 book automatic download while the Kindle required me to download them individually.
I will admit that there might be way to force download all on the Kindle but that is not in an easy to find location and a quick web search did not find anything other than display all books and tap on them to download.
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