Why are the Thumbnails not showing up for all episodes after adding TV Show L.A. Law to MCM?
The only work around for this is to use Automatically thumbnail extraction on each episode then Fetch metadata.
I'm not going through 171 episodes to fix this, been an issue for sometime now and no-one knows what is going on!!!

What exists is the seasons hidden metadata folder? Are the images detached or just missing?

Actually TVDB has no images for this series and MCM has to extract images for every episode.

But there is a outstanding issue with MCM and image extraction. I call it a race issues as MCM pops up with the extraction but it actually does not happen. You can see this with manually trying to extract for multiple episodes also. So it is tedious doing one by one at a slow interval.

The seasons hidden metadata folders only shows up the .xml files and no images, if I was to manually extract a image and changes from the highlighted yellow to normal then the image will show up in the metadata folder.
At least Plex is showing the images for each episode.
Fair enough, if they are not listed on TVDB they will not automatically come in.
Hopefully MCM can be fixed to allow it to take over image extraction if the above was the case!!