Cornerstone only uses the Full-Size image in the Image Element so I wonder what could cause that. I see you’re using the latest Standalone version of Cornerstone (4.1.2). I use that in my test site with TwentyTwenty theme too and the issue does not happen on my end.
There could be another plugin that could be causing the issue or overriding the image. Would you mind testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If that doesn’t help, copy your live site to a staging server and test using a different theme. It could be that there’s a function in your theme that is affecting Cornerstone.
Please also give us WordPress Admin and FTP access to your staging site in a Secure Note