How to set up thumbnails correctly in folder view ?

[phantomwise]

So I tried a Music Videos library... and when the scan started it crashed my NAS . Like completely unresponsive, even the power button wouldn't shut it down and I had to manually unplug it.

After restarting the NAS and rebuilding the docker project however, the Home Videos library started to display thumbnails correctly...

I waited a while to test new libraries of different types because I wasn't too keen on crashing my NAS again, but I finally made three new folders with the exact same files for three library. And it didn't crash this time !

Mixed Movies and Series :
   
Still shows the same thumbnails for every video in a folder... wish I knew why.

Home Videos :
   
Now displays correctly !
The thumbnails aren't displayed as image files in addition to the video anymore, even with "Display the photos" checked in the library options.
I don't know why it did that before, it was weird... I can't remember if I had tried to rebuild the docker container before so maybe that's what fixed it ?

Music Videos :
   
Also displays correctly !

Thank you for the suggestions, I wouldn't have thought to try a different library type and now it works even if I'm not sure why