i think i found the issue – for some reason the following action is not being fired on the front end:
add_action('post_edit_form_tag', array($this, 'post_edit_form_tag'));
as a work around i use the form_attributes argument to the acf_form function when i create my front end form and it works perfectly
'form_attributes' => array(
'enctype' => 'multipart/form-data',
),