I've had this problem as well. When going from offline to online, it removes books sideloaded via USB. But not books I've added via send to kindle or ones that

I've had this problem as well. When going from offline to online, it removes books sideloaded via USB. But not books I've added via send to kindle or ones that I've downloaded via the experimental browser. So now, instead of sideloading via USB, I convert to mobi, run a python -m http.server on my laptop and use the experimental browser to download them on kindle.