The content field on the front end uses an ACF WYSIWYG field and not the standard WP field.
You might be about to alter the default value using an acf/prepare_field function… but this is only a guess and not tested, so I don’t know all of the details. https://www.advancedcustomfields.com/resources/acf-prepare_field/
add_filter('acf/prepare_field/name=_post_content', 'set_acf_form_default_content');