Hi there,
This is not related to the theme and cornerstone update, the issue is because The Grid’s content is being placed to Essential Grid and replacing what’s in there. And something is doing that, so probably your calculator. Have you updated the price calculator? Or maybe it’s due to cache, and disabling one plugin could clear or regenerate the cache.

Do your site has CouldFlare setup? I remember SiteGround has that built-in. Would you mind providing your login credentials for that too? But yes, the issue is between Essential Grid and your calculator, maybe ESG has new structure that has conflicts with the calculator. Maybe the calculator is performing WP queries too?
But maybe this is script related too and we need to find out first if the script that being served is from CF.
Thanks!