That does sound like there is something failing to work with the subscription on service of Tableau Server. I was doing some research on the issue and was unable to find any known issue for the behavior being experienced.
To better assist can the following information please be provided. Tableau Server version and any errors that might be received during when the subscriptions run. I would also be curious to see if the view is truly blank or if something is preventing the image from being loaded in the email. e.g. Email security client of some sort.
Hope this helps and look forward to hearing from you!