Had the same issue. Contacted Apple Support (via Twitter) and tried every one of the above fixes, all to no avail. But, as above, this solved the issue:
1.) Adding movie file to iTunes via the local hard drive
2.) Adding artwork
3.) Coping updated movie file to iTunes NAS
Strangely — and this is where I first noticed the problem — I had the same issue with the iTunes Store download of The Force Awakens. I downloaded the movie directly to the NAS— my first time doing so — and, mysteriously, the artwork wasn't included. Tried every which way to add it and had no luck. Eventually, mysteriously, the process worked, but I had already watched the movie twice from start to finish. So, I assume that the "Run Movie To The End" fix worked in that instance. However, in the latter — a legally ripped Bluray scrubbed through to the last 3-4 minutes, then run out — it did not.
In any event, if your iTunes media folder is located on a Network Attached Storage device, you may have trouble adding artwork or receiving packaged artwork from the iTunes Store. One of the two above procedures seems to fix the issue, depending on how the media was created.
(Editor's Rant: Given the increasing preference for digital movie files, and their relative size respective to most hard drive capacities, it would be preferable if Apple provided better support — or any support — for using iTunes over a network. I keep my movie files in a separate library for this reason and Option + Launch to switch between the two. This is Apple's officially recommended method and, save for the issues outlined above, it works. It's a bit ridiculous, though, that in 2016, I still have to restart a program every time I want to change a simple directory setting, especially a program as frequently accessed as iTunes.)