I've been having the same issues, and managed to resolve it via the following:
1) Select all TV shows, and Bulk-Change tags ('Get Info' with all shows selected), clicking the checkbox but leaving the field beside it blank for all 'Sort XXX' fields
2) Setting the Episode ID value to a unique value *** ACROSS ALL TV SHOWS ***
This 2nd step I believe is the critical one, but it's possible the first was required too.
Unfortunately, just leaving Episode ID blank was not sufficient.
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I did some experiementation and found that if you set the episode IDs unique across just SHOWS then it didn't work. e.g.
Episode ID: 1 Show: TV Show A Name: A walk in the park
Episode ID: 2 Show: TV Show A Name: Up and down
Episode ID: 1 Show: TV Show B Name: Skateboard Park
^^^ this would result in eratic display of artwork and shows
If however, you set the Episode ID uniquely across ALL shows it worked. e.g.
Episode ID: 1 Show: TV Show A Name: A walk in the park
Episode ID: 2 Show: TV Show A Name: Up and down
Episode ID: 3 Show: TV Show B Name: Skateboard Park
^^^ Note the "3" being the 'first' episode of TV Show B
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For large libraries this is a pain, but I found this utility which can do it quickly with the right usage:
http://dougscripts.com/itunes/scripts/download.php?sc=incrementnumbertags
1. Install it as per instructions
2. Open iTunes
2. Select ALL TV shows (switch to list view)
3. From 'Scripts' Menu in iTunes select 'Increment Number Tags'
4. Tick 'Episode ID' leaving default of '%n' in the field, and leave the other checkboxes unticked.
5. Click OK
NOTE: This will OVERWRITE your existing Episode IDs
This should go through all TV shows, and change the Episode ID value to a numeric value of 1 ... 2... 3 ... from the top to the bottom; thereby making the episode field unique.
If you add more TV shows, you will likely need to repeat this process to ensure the newly added entries have unique, non-blank values.
Hope this helps.