Slightly chuckling here because I seem to remember a few months ago that people were complaining that when they bought stuff it no longer went to their default kindle but their 'library' and then they had to go to Manage Content and Devices and send it manually.
OR that the default kindle kept getting un-set. That used to happen to me but I haven't noticed it in a while.
I can say for sure that currently library books go to my default kindle and there's an opportunity right on the page at the time to then send it to other devices if I want. I feel like the same is true for purchased books but I don't do that as often so can't be sure I am remembering correctly.
If I download a KU book on my phone or tablet, that's the only place it ends up. Buying via Kindle apps is no longer supported.
If you're in 'home' view on a newer kindle, each new acquisition is going to be on the top banner and, unlike in the android app, you can't remove it from home. All you can do is download it or un-download it, but it stays in that banner. For me, that means it's cluttered with my daily newspapers so becomes not very useful; hence I keep it in Library view.
In Library view, you do have to watch whether you have it showing Downloaded or not. If not, you'll see everything you acquire whether it's on that device or not.
I guess, in the grand scheme of things, these oddities don't bother me that much. As long as I can find and read a book when I want to, I'm happy.
