[Ubuntu 14.04] System randomly loses posters/artwork for movies, possibly due to a custom scanner?

Ubuntu 14.04, download the plexmediaserver_0.9.12.19.1537-f38ac80_amd64 db file to install it.

I previously wrote my own roku channel and had my movies named specifically for that, in a format like 123.movie_title.mp4, where 123 was the ID from themoviedb.org and I used their API to pull all the meta data.

Recently, I moved over to using plex as it seemed easier than adjusting my channel further. However, I didn't want to have to rename all my files. Thus, I created a custom scanner that sets the guid to the movie based on the ID in the title, basically like so:

# Scans through files, and add to the media list.
def Scan(path, files, mediaList, subdirs, language=None, root=None, **kwargs):
    VideoFiles.Scan(path, files, mediaList, subdirs, root)

    for i in files:
        file = os.path.splitext(os.path.basename(i))[0]
        guid = "com.plexapp.agents.themoviedb://%s?lang=en" % int(file.split('.')[0])

        (name, year) = VideoFiles.CleanName(file)
        movie = Media.Movie(name, year)
        movie.guid = guid
        movie.source = VideoFiles.RetrieveSource(file)
        movie.parts.append(i)
        mediaList.append(movie)
    Stack.Scan(path, files, mediaList, subdirs)

This worked well, as when I set a new library to use this scanner, it reads the guid and loads all the metadata from themoviedb.org. However, I noticed that days later... without rebooting or the server restarting or anything, movies just start randomly losing their poster and use a frame from the movie. All the other metadata seems to be there still, it's just the artwork that vanishes. Doing a reload fixes it again, but not permanently.

I'm not sure where to start debugging this, and it's getting very irritating. Any pointers?

Looking at the logs, I don't see much that looks relevant, stuff likethis shows up though:

com.plexapp.agents.themoviedb.log.5:2015-12-11 21:38:03,139 (7fb3c67fc700) :  ERROR (model:205) - Cannot read model from /var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Metadata/Movies/3/0003e0716643d8767d86320e9a341f15edbc6b6.bundle/Contents/com.plexapp.agents.themoviedb