Hi James I came I use the following filter I get the same issue:
add_filter('acf/load_field/name=hidden_post_id', 'make_hidden_post_id_readonly', 10, 1);
function make_field_readonly($field) {
// sets readonly attribute for field
$field['readonly'] = 1;
return $field;
}
the hidden_post_id disappears from the field group in the acf menu.
I actually copied that filter script from John Huebner:
https://support.advancedcustomfields.com/forums/topic/accept-only-unique-values/#post-45352
