How do I disable the Mac App Store notification badge for successful updates?
I believe it’s a bug (not sure what would make it an “obscure bug”) and you should submit feedback to Apple at https://www.apple.com/feedback/macos.html
The reson I believe it to be a bug is because if you immediately open the App store, the erroneous badge immediately disappears meaning that upon launch of the app, it correctly indicates that there are none.  In fact, it appears that the badge is “reset” as it checks for updates with the update server.  I have personally seen mine go from 4 updates, then upon opening the App store, the badge disappear and then a few seconds later change to 1 update.
It’s entirely possible that the badge doesn’t update until the next automatic update check.
I don’t believe this to be intended behavior and obviously a nuisance, but it’s nothing that affects functionality.  You can easily fix the badge by opening and closing the App Store app.  I submitted feedback regarding this and I encourage you to do so as well.