Confirmed by your support team. This isn't a bug, it's "by design". See below screenshot.
Here's the scenario: I have a couple of hundred books organized in different Collections. Right now my wife needs to browse past a bunch of sci-fi books she has no interest in (*sigh*) to get to the books she likes. And that's where Collections could save her a ton of time!
So, my request (P1) is that you allow Collections to be displayed/browsable by household members. As a P2 Requirement, I'd like to members to be able to sync a whole Collection to a given Kindle with one tap. If that's not too much to ask :). Thank you! ❤️
@AmazonKindle please add the ability to show Collections for non-download books for family account. Ability to share large number of books with spouse/kids is basically broken right now 😥