I just wanted to clarify that "Download and Transfer over USB" going away only means you can't download official kindle purchases and then transfer over USB. If you have ebooks purchased elsewhere you can still transfer them over USB, I still use Calibre to transfer books to my kindle PW5 SE.
I haven't had it happen recently but I have had side-loaded books disappear in the past on my PW5 SE (this is 2021 model in case I am messing up the numbers). If I recall it was only an issue when I kept it in airplane mode for a long time and then when I connected to internet again they were removed. My understanding is there **should** be a way to mark them as documents or something and have them stay but I have not looked into that at all so I unfortunately don't have much info there.
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