I too have noticed that after installing iOS 8.0.2 the thumbnails aren't showing up on the iPad mini. In my case it's about 50 home videos that I have created using iMovie and exported to iTunes.
What works for me is that for every single clips under 'Get Info', I change the 'Media Kind' for all these clips in iTunes to : Home Video.
Now, the thumbnails are showing again!! No thumbnail editing required, just re-categorizing...
Cumbersome, annoying and having to re-sync all movies to his iPad, but at least grandpa can now see his twin grandsons' clips again, whilst browsing through the thumbnails! I did have to point him to the Home Video-section in the Video app on the iPad, though…. 😉