LG webOS 3.0 and 3.2 - detailed list here - are not capable of sending screenshots while you play a playlist of FullHD videos in a gapless manner. It's a webOS limitation that results in not gathering screenshots and not be able to show them in the Box or provide them via API.
Solution
Unfortunately, there is no solution to this issue. It's connected to the hardware performance and limitations that cannot be overcome unless you opt for non-gapless video playback or you play videos with resolution lower then FullHD.