<<<I'm capturing HD>>>
Someone else will jump in, but just based on the fact that this screen never fails under normal circumstances, I can conclude that it's supposed to be that way.
The default for standard definition captures is for the capture window to show you about eight to ten frames per second video and no audio. Yes, you can force the system to give you audio, but it's roundly considered a bad idea.
Therefore, I would not be shocked to find out that the capture screen finally threw in the towel and doesn't show you
anything during live HiDef capture in order to devote as much energy and talent as possible to the actual show.
Koz