Hi,
When I duplicate a template and make changes to it the thumbnail is actually updated and reflecting the changes. Can you tell me what the exact Acymailing version number you are using?
And can you try a different workflow to check if that's working better:
- Click ion the create button to start working on a new template
- Click on the button "Start from" to select an existing template and pull in the data in your new template
- Make some changes
- Save the template and check the thumbnail
