That prompt appears because it's the first time you're using that version so you might want to check the release notes. So it prompts to ask: do you want to view the release notes? I see that to a different case to a "new version is available" prompt.
So it does know what version I was using previously. Why can't it prompt me?
I'm confused what you expect it to prompt you? Previously you were visiting editor.construct.net, and it did prompt you to tell you there was a new beta version available, because you'd chosen that in your settings. It offered options to keep using that version, or switch to the newer version, just like you suggest. When do you think it should prompt you instead and what would it say?