Autosave empties Custom Post Fields

The save_post action runs on autosave as well, but autosave doesn’t post back custom fields. You need to check against wp_is_autosave($post_id) and wp_is_post_revision($post_id) before updating the meta values.

Strange, the function wp_is_autosave()doesn’t seem to exist

I guess what you need is DOING_AUTOSAVE: (from https://www.andrewgail.com/wordpress-autosave-and-custom-fields/)

global $post;
if (defined(‘DOING_AUTOSAVE’) && DOING_AUTOSAVE) {
return $post->ID;
}

gotta love how the dev’s claim that the autosave functionality is a ‘stop gap designed to prevent losing data’, when this behavior is doing exactly that – losing data.