It's a new bug, which is odd, I didn't notice there being an update or anything.
In the past, you could use the builtin chromecast as a chromeccast. But now, if you do that, the chromecast background will stay visible. You can hear the video and everything, but the video won't appear unless you hit the home button in the mibox remote.
This really kills the strongest point of android TV for me. That not only you can run any android app but that it can still function like a chromecast when you just want to play video from your phone :/
Ive heard a lot of people solved their Chromecast issues on Android TV by uninstalling and reinstalling the Chromecast built-in app from the Google play store