Recent new features in Shopify can lead to parts of your theme not displaying to users based on the location they are in.


What is confusing - is that in Theme Customisation preview - everything looks great! Which provides incorrect feedback on what is occurring.


This is due to having different Theme variations based on your stores market settings. 


https://help.shopify.com/en/manual/online-store/themes/customizing-themes/store-contextualization/understanding-markets-and-overrides


You can see these (if you have them)



There are basically two options to resolve


Resynchronise the section - follow these instructions to reset the Page template back to your standard


https://help.shopify.com/en/manual/online-store/themes/customizing-themes/store-contextualization/create-and-manage-overrides


The final option is to select the market variation in your Theme customisation or via navigating by your Markets menu. Editing the theme to add the app block or modify it and then saving it


Shopify are aware of this - and are working on a fix


https://community.shopify.dev/t/app-block-getting-excluded-on-page-render/14672/20