XenForo uses whatever comes through in $_SERVER['REMOTE_ADDR'] -- this comes directly from whatever is executing PHP (FastCGI server, Apache, etc). If they're doing a reverse proxy, then yes that may change the IP given to PHP. You should be able to demonstrate this trivially with a simple PHP info file (<?php phpinfo(); ?>).
You can override this in config.php if the original IP is actually forwarded in other header, but you shouldn't have to if it suddenly appeared and your host changed something -- ideally they would sort it.