According to the documentation:
"The Generate Thumbnails as User option in the publish dialog box is only available when the workbook contains one or more active user filters."
That being said, if there aren't any user filters, you should see a thumbnail correctly.
What are your datasources like? If they are server-hosted, are they the same in production as in dev/stage?
I am seeing a blank thumbnail when i am publishing without a user filter. its supposed to show some thumbnail in server even if we publish it without any userfilter.