So far it is working with just HTML, but not with {{ product.metafields.custom.add_to_cart_button_fact.value }} or {{ product.metafields.custom.add_to_cart_button_fact }}
Are you using it in the product page or somewhere else ?
Instead of this {%- render ‘customer-box’ -%} try to use {{ product.metafields.custom.add_to_cart_button_fact.value }} or {{ product.metafields.custom.add_to_cart_button_fact }} in the same liquid block and let me know but make sure you are in the product template.
I am using it on product pages, I did a triple-check. I have several different product templates, but I made sure I am opening the correct product with the corresponding product template.
I tried both {{ product.metafields.custom.add_to_cart_button_fact.value }} and {{ product.metafields.custom.add_to_cart_button_fact }} in the same liquid block, but still empty although I have an input (see screenshot).
Sounds like shopify provides a different dashboard to you lol
btw if you want you can provide the store access so I can check and let you know what is wrong there.