Hi there,
I understand, theme issues can be difficult to solve. Our support is extremely limited on 3rd party themes and plugins. Since it works with most major themes its hard to say exactly what the issue could be. Next steps would be to reach out to the theme developer and give them the same example, they may have other reports of thumbnails not working after an update.
Also, if images are not showing up, you may want to install and run the “Regenerate Thumbnails” plugin (not our plugin so use at your own risk). However, I would do this in a staging site first as referenced in my first message to ensure it solves the problem.
Let me know how that goes.
Thanks,
Brendan