I was unable to recreate your issue when using a Thumbnail Grid in the Footer with the hover image option "on".
Though i do agree that when editing the Thumbnail grid the image content disappears if that is a bug?
If debugging please make sure of the following:
Wordpress, Lay Theme & Laytheme's Addons are all up to date ?
Any custom CSS or JS that may interfere is temporarily removed.
Any third-party plugins are de-activated to see if this resolves the issue.
Best wishes Adam and have a wonderful day 🌝