I did some more looking and I think this has something to do with acf_form_head(). The field _post_title is defined in acf_form_head(), or at least the function that calls them is called by acf_form_head(). This is what I was looking for because of the fact that all the attributes are empty let me to believe that the field was not properly defined. Try calling _acf_form_register_fields() at the start of your ajax function before you call acf_form()
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