As reported in my G+ posting if I have Internal Player unchecked and choose a Chromecast target to play to, the FE will still try to play to an external player.
If I go check the Internal Player item in settings and try to play the same content, I get a dialog about that item not being playable via Google Cast.
To be clear, the bug report here is that not having Internal Player selected seems to override one's Chromecast selection and still wants to open the external player for playing.