I'm not sure what would cause it, but does republishing a workbook from Tableau Desktop cause the thumbnail to show again?
Thanks for you interest.
No, republishing doesn't cause them to show.
All workbooks and views are like this (new, old, republished)
Wow! I just double checked, and I can't think of or find any settings that would cause that to happen. I recommend opening a case with Tech Support so they can investigate directly.
Thanks for your suggestions. I read this post already.
I have no user specific filters anywhere on the Tableau Server environment.