This has been happening to me as well. Whenever I power on my Kobo (Sage, just recently updated to 4.37.21533), out of 4 sideloaded books (I use Calibre), only one remains. When connecting to the computer and Calibre, file explorer shows the books on the Kobo. When I disconnect, (without using send to device in Calibre) I get a message as though it uploaded the books again, and the books reappear on the Kobo. Next time I close and then reopen, same thing happens.
I have not changed anything about how I use the Kobo (this one and all previous Kobos I've owned over the past 5+ years), and it really started after I updated the software.
Will try repair, just in case
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