I'm working on the migration from ES2 to AEM and it seems that there are some problems mainly with the "Preview HTML5" TAB in AEM.
In designer AEM, when I tried to add a new instance of a repeatable subform (using addInstance function in a click event of a button), the new instance shows in preview PDF, it doesn't in "Preview HTML5" .
It seems that is related to a refresh page problem.
With the HTML5 version of the form, you need to have at least one instance of the subform that exists initially. Also, make your life much simpler by ensuring every element in your form has a proper name including subforms. You should have nothing that is "untitled". Keep in mind that in the browser "app" does not exist so you should try / catch that if you'd like to use it in web & pdf.