This happens because DoCast prevents the screensaver from appearing on Fire TV. To prevent the loading indicator from showing, you need to:

  1. Disable the screensaver in the Fire TV settings:

    Display & Sounds > Screensaver > Start Time > Never

    Note: By default, Start Time is set to 5 minutes.

  2. Turn off the "Disable Screensaver" option in the DoCast settings.