This is an unfortunate side effect of some changes we made for the Cornerstone 5 update. Prior to Cornerstone 5, every element stored the full URL to the actual media library asset. When Cornerstone 5 was released, all newly created elements that reference images will store the attachment ID instead of the URL. This allowed us to have better integration with the WordPress media library but now template exports don’t have a way to locate the images anymore - because it just sees numeric IDs instead of URLs.
This is something we are planning on addressing but is a bit involved so we haven’t been able to prioritize it just yet. I don’t have an ETA on when it will be addressed. Meanwhile, to ensure media assets transfer you can use the Insert via URL function when adding an image and copy/paste over the image URL. I understand this is not ideal but regretfully it’s the only way to keep images portable at this time.