Hi Dave,
It could be a compatibility issue but it’s not something that can be fixed easily since two developers (our site and their side) should handle it. All we could do is add this to our issue tracker.
But based on my understanding of the code, the theme and cornerstone don’t alter the image URL supplied by https://developer.wordpress.org/reference/functions/get_the_post_thumbnail_url/ (or similar). The plugin uses it directly without dependency on the theme and cornerstone.
I’ll do some further checking if you can set up another installation (staging) without the unrelated plugins. Just make sure the present plugins are the cornerstone, Woocommerce, and wholesale-related plugins. And of course with X theme too. I tried this on your current setup by deactivating the others but it only causes internal errors. I can’t promise any fix, but if I can confirm this incompatibility then I’ll add this to our issue tracker along with any information that I’m able to find.
And yes, updating cornerstone is not related but you must update it since you already updated your theme. This will also help us narrow down the issues, and to make sure the issues aren’t coming from other areas (like an incompatibility between X and Cornerstone)
Thanks and let me know.