A part of page missing after publishing is a critical error. Unfortunately it may happen to you in some rare situations.
The most common reason is page too big. Shopify and BigCommerce have their own limit at how big a page can be.
This error happens more frequently on BigCommerce Category pages, because BigCommerce is very strict about their length limit.
Unless you are designing a huge long page, this error should not happen to you. But if it does, you can consider removing some parts and try again or contact our support team for help.