Well, I don't know why your experience sideloading AZW3 would be different from virtually everyone else's.
Details?
As for Amazon sending ebooks, the vast, vast majority of my books download as AZW3, and I assume the odd MOBI was published
and uploaded before AZW3 was a thing.
Amazon prioritizing MOBI over AZW3 would imply that AZW3 was not ever actually used or sent to peoples' devices, which is just silly.