Thanks for posting in! Are you using CloudFlare or may have installed a caching plugin like WordFence, W3 Total Cache or WP Super Cache? Always remember to clear all caches when updating so that the code from the latest release is always in use. This will help you to avoid any potential errors. If your server has a caching feature, you might need to flush the cache as well.
We could not certainly tell what is causing this issue in your site since we haven't investigated it yet. This can be cause by a 3rd party plugin or something. To know more about this builder preview message and how you may resolve this, please check this out:
Hope this helps.