I’m also having similar issues, had written some simple code like your self as I suspected these were not firing. I have tried.
//save_post only fires when you create a new post NOT the first save / publish
add_action('save_post', 'save_postdata');
// never seems to fire
add_action('edit_post', 'edit_postdata');
//never seems to fire
add_action('publish_post', 'publish_postdata');
// fires after save_post above
add_action('wp_insert_post', 'insert_postdata');
I have written some custom write panels for my client as detailed in https://wefunction.com/2008/10/tutorial-creating-custom-write-panels-in-wordpress/ but can’t get them to save ??
Any ideas
Rob