On further inspection it seems to be something to do with the following filter action which is called in the process_value function in inc/field.php of the MB plugin.
$value = self::filter( 'sanitize', $value, $field, $old_value, $object_id );
Prior to this filter the submitted $value (in this case 'THEREG') is correctly defined. After this filter the $value is empty.
What sanitizing is happening here and how can we bypass it or meet its conditions?
Cheers,
Will