My apologies. The plugin works as designed.
I’m not quite sure why but when I moved my metaboxes from an admin_init hook function in my theme functions file to the global scope everything worked fine. The problem was with the AJAX call, but admin-ajax.php makes a admin_init hook call. So maybe it’s not called in time? I’m not sure. I guess loading my metaboxes even on the front end isn’t a problem.
