Excerpt text not showing in meta box, causing flush on update?

I've created a custom post type that supports excerpts. On a post update of this custom post type, the content in the textarea for the "Excerpt" meta box is saved, however it isn't output into the textarea meaning that any further updates flush the saved excerpt.

For reference, these are the arguments of the custom post type:

$args = array(
                    'label' => 'Projects',
                    'has_archive' => true,
                    'publicly_queryable' => true,
                    'show_ui' => true,
                    'supports' => array('title', 'editor', 'thumbnail', 'excerpt'),
                    'taxonomies' => array('post_tag'),
                    'rewrite' => array('slug' => 'projects')
                );

I can't think of anything in particular that could be causing this, other than perhaps a faulty WP core build. I am not re-using the ID used by the metabox for excerpt anywhere.

Any ideas on how to resolve from similar and/or other experience would be greatly appreciated!