I have the same problem. it happens only when i submit a post from the frontend as a custom post type.
this is the code i use for the submitting form in the frontend.
//Post Properties
$new_post = array(
'post_title' => $title,
'post_content' => $description,
'post_status' => 'pending', // Choose: publish, preview, future, draft, etc.
'post_type' => 'index' //'post',page' or use a custom post type if you want to
);
//save the new post
$pid = wp_insert_post($new_post);