Well, I found a workaround for this:
add_action( 'init', 'my_custom_rest_support', 25 );
function my_custom_rest_support() {
register_rest_field( 'my-cpt','long_description',
array(
'get_callback' => 'get_long_desc',
'update_callback' => '',
'schema' => post
)
);
function get_long_desc( $post_id ) {
global $post;
$post_content = get_post(get_the_ID());
$content = $post_content->post_content;
return $content;
}
}
So even though this is not showing twice in the admin, I can nevertheless add the field to the json REST response. Hope this helps someone else out…