I've found that when generating the trick play playlist, that the roku is very particular about the LAYOUT attrib on the last tile for the EXT-X-TILES tag. If not all the rows or columns are filled, then they need to be decremented for the trick play images to be displayed properly. I don't think that https://github.com/rokudev/samples/blob/master/media/TrickPlayThumbnailsHLS/scripts/gen_playlist.sh handles this properly.
I've found that when generating the trick play playlist, that the roku is very particular about the LAYOUT attrib on the last tile for the EXT-X-TILES tag. If not all the rows or columns are filled, then they need to be decremented for the trick play images to be displayed properly. I don't think that https://github.com/rokudev/samples/blob/master/media/TrickPlayThumbnailsHLS/scripts/gen_playlist.sh handles this properly.