You're trying to extrapolate on this specific scenario from Wikipedia pages. Have you done any of this work? What have you done when you've reported a vulnerability to an upstream with dozens of downstreams? When your teammates have? You keep talking about "protocols" and "commonly followed practice" and "codes of ethics". Tell us more about the codes, protocols, and practices in your shop.

Nobody, for what it's worth, is arguing that major distros shouldn't have gotten some kind of notice. The problem is that the entity responsible for doing that isn't the vulnerability research lab. In fact, as a general procedural point, researchers can't go contact downstreams. They might be able to do so in the specific case of Linux, but you've tried to spin that possibility into a binding obligation derived from established practices, which: no. That's not a real thing.