Custom metabox fields on user object types have stopped saving in v2.0.9. Best I can tell, it's because of the introduction of can_save() in CMB2_hookup.php without the object type being passed.

It works for me after changing to:

if ( $this->can_save( 'user' ) ) {
    $this->cmb->save_fields( $user_id, 'user', $_POST );
}

Happy to send a PR if you agree with the solution and that'd be of help. For now, rolling back to v2.0.8 fixes this in my case.