bug is fixed in SVN 27167
Hmmm, just tested it. Now there is sometimes a video preview - the first time entering BasicHome the running TV/Video is shown. Entering Videos in list view, nothing is shown (as it should). Going back to BasicHome, now nothing is shown.
Going to videos and changing layout to Covers or Coverflow (where it is allowed), preview may be shown, but mostly it is not shown. Now it´s very random....
EDIT: Now I (possibly) found the reason: Opening the action menu displays a flickering overlay, and depending on when action menu is closed, the overlay displays (or not)