I haven’t been able to replicate the problem. But if you’d like to send a login I’ll see if I can trace it in your system (or a staging system with the same problem). You can contact me here.
You can also check for theme/plugin conflicts:
1. Deactivate ALL other plugins and switch to the standard “twenty twenty” theme.
2. Create a test post with a thumbnail
3. Check if the new thumbnail is displayed on the mashup