Do you use Hardware 'External FX' in your mixdowns?

I’m not exactly sure what you mean by this statement?

If you have your buffer set to something like 512 and you are not direct monitoring via another device/interface (which I would recommend) and instead count on Cubase monitoring in the box. Then you will certainly hear a latency if playing something and monitoring. But your external FX Ping can still read 0. They are not necessarily related.