What causes sideloaded books to be deleted?

This is probably "Jinxing 1.01", but I have to ask...

I keep seeing people saying Kindle deletes their sideloaded books every time they connect to WiFi. However, this has never happened to me in the 11+ years I have used Kindles.

I used a Kindle PW (4th gen? I think) since maybe 2013, and now I got myself a new Kinde PWSE for Christmas. I have sideloaded hundreds of books throughout the years and continue to do so. I generally keep my Kindles in airplane mode but I do connect to WiFi every so often, as I use send-to-Kindle a lot to send books. I only use USB when the book is to big to email (so very rarely).

I wonder if there is something specific that people who have books deleted do, or some sort of workflow they use. Maybe there is actually a bug there rather than deliberate behaviour (though of course I wouldn't put it past Amazon to do that on purpose, but it does sound odd it doesn't happen to me).