Add Meta Box not showing on New Post – Custom meta box not showing

I would create the instance of the class on init or even admin_init and not on ‘load-post.php’.

Hooking into load-post.php will not add the boxes to NEW posts and only posts that are already saved. The correct hook to add meta boxes to ONLY new posts is ‘load-post-new.php’.

If you want to have the meta box display on both new posts and existing posts, use the ‘add_meta_boxes’ hook.

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