I know other Android TV users were complaining about ads on the launcher, but on Mibox3 it just breaks navigation.
My older Mibox3 received the update, after which you could only navigate left/right on the app row, but you couldn't navigate up/down to other rows or even settings.
To fix: I used this airmouse app from my phone, went into Settings > Apps > System Apps > Android TV Home, and uninstalled updates. You'll then need to disable automatic PlayStore updates, and sideload v2.3.1 of the app.
Thanks for the heads up, I was thinking about updating it.