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Regarding usb loaded/sideloaded AZW3 books I've seen these problems reported:
1) Page numbers don’t work
2) % read tracker doesn’t work, always said Read 100%
3) Painfully slow page turns
4) huge margins on manga/comics at the top and bottom of the screen.
(Another 5.19.2 bug where a "Storage is full" error would constantly appear was apparently fixed rather quickly though.)
And apparently logging out of the Amazon account on the Kindle fixes all the problems.
Not everyone will experience the problem, Amazon can remotely activate features so it's possible the "feature" hasn't been activated for you yet.
Sideloading KFX format books doesn't seem to be affected.