When playing a Matroska video file ".mkv" video snap, the video plays file plays fine, but then the box art for the game disappears with ffmpeg. It works fine with Windows Media Player with K-lite installed. .mkv is also not shown as a valid video type on the video snap selection screen when selecting a video snap. You have to select "All Files." I hope this helps.
Edit: Selecting Windows Media Player, OK, then going back and selecting ffmpeg fixes the problem, but the problem reoccurs if I select another mkv video snap.