How do I force a CPT to select a shared custom template in Post Attributes?

I have a custom templates that is used by several custom post types. This code forces the CPT singles to display on the front end using it:

function get_custom_post_type_template($single_template) {
  global $post;
  if ( in_array(get_post_type(), array('custompt1','custompt2','custompt3')) ){
    if ( is_single() ) {
      $single_template = get_stylesheet_directory() . '/single-custompt.php';
    }
  }
  return $single_template;
}
add_filter( 'single_template', 'get_custom_post_type_template' );

The problem is that in the admin the Post Attributes multiple select input still shows Default Template.

I need my custom template to be selected in order to get ACF fields to display (I know I could simply make the ACF fields display for each CPT but there are quite a few CPTs andI decided to use the CPT template as my criteria). Hiding the Post Attributes completely would be fine as long the right template is selected in the admin.