I saw this posted on the old forum. The issue is that someone thumbnails get generated and sometimes they do not.

Debug prints:

06.22.2020 3:05:26 - upload/twg_helper.php:/home4/drakesc3/public_html/gallery/ffmpeg -y -i "/home4/drakesc3/public_html/gallery/pictures/Collections/Dragons/Plush and Stuffed/Drake Tail V2/Drake tail V2.mp4" -ss 1 -vframes 1 "/home4/drakesc3/public_html/gallery/pictures/Collections/Dragons/Plush and Stuffed/Drake Tail V2/Drake tail V2.jpg"

(modified, the command-line a little.)

Turned out, the issue was with the ffmpeg version I used. Originally the server reported back as x86_64 so I loaded the amd64 version, the current version in my case was i686 release:

https://ffmpeg.org/download.html

It seems to be parsing and creating thumbnails on bigger files fine.