Subform fields showing in pre-made templates, but not in custom HTML templates.

I am attempting to get subform field information to show up in my custom templates, but the fields are not showing. The other day I created a template based on the Zoho basic pre-made template and the subform fields were showing information.

My question is, is there a way to get the subform field information showing in a custom HTML templates, and if not, why is this feature being restricted to the pre-made only templates?

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