That makes sense. You aren't really "updating" at all. You're basically reinstalling a new Chrome on every update. It makes sense then that you aren't seeing "what's new" because that's not how a fresh install starts up.

Note that this package and update is actually not maintained by Google at all, it's done by Gentoo: https://wiki.gentoo.org/wiki/Project:Chromium/How_to_bump_Ch...

I hate to be an apologist for anything but I think you are pointing fingers in the wrong place. The Google-official releases use the built-in automatic updater and do show What's New. This is a Gentoo release and they chose to do their own thing for updates.