add_meta_box Callback not being called

Looks like we both missed the error because I didn’t see it at first either.

In your call to add_meta_box you skipped the argument for context and went straight to priority. “high” is not an available string for the context argument, and this apparently causes the function to silently fail.

Adding in the string for context fixes it:

add_meta_box( 'package_meta', __( 'Package Title', 'package_textdomain' ), 'package_meta_callback', 'package', 'normal', 'high' );