This is really bizarre. It happens a lot to me too, but I can't figure out why. I don't know enough about myth. What happens is that the MythTV_Player, which is the LMCE program inherited from Pluto, is really just a wrapper that sends commands to the myth frontend in a socket. mythfrontend is just spawned like normal. This is not a modified version of myth in any way. If you tail DCERouter.log, you'll see the command Spawn Application is sent to App Server, which is the device that does the spawning. And, as someone pointed out, whenever app server spawns an application, that apps output is logged to Spawn_[pid]_[name].log. So, you should see the logged output for myth frontend.
Here are the things I've been looking into: apt-get install wmctrl and then use wmctrl (window manager control) to confirm that mythfrontend's window is active. As far as I can tell, it is, and the gray/black screen is actually the myth front end. One could also kill Orbiter (killall OrbiterGL or killall Orbiter) to see if someone Orbiter was hiding mythfrontend; note: when you kill Orbiter, it will automatically restart in about 10 seconds.
When the spawned mythfrontend process exits, App Server sends a commands back saying the spawned app terminated, and that's what causes lmce to know that tv is over. I'm going to be working at this more intensely as I'm trying to get lmce 1.1 out for feisty, and it fixes a ton of issues and has a new installation method.