I have some videos (home movies, tv episodes etc.) that are just on an NFS share as files, not part of the kodi library, and most of them don’t have thumbnails generated despite enabling the setting.
Oddly enough the single x265 file I have does have a thumbnail, none of xvid/avc files do.
The same files all show thumbnails on my Atom/Ubuntu box running kodi 18b3 (4k+ is running 17.8-335)
Its not a permissions problem as the videos play fine and i can read them using mediainfo.
Edit: hmm, seems to be working now, looks like a bad rsync from my “master” kodi box - Thumbnails dir was only 38mb not 3.3gb! bit odd it didn’t re-generate them though.