I am guessing the Left, Right, Middle designation is just how EVGA does it and on my 1070Ti the Middle Fan / header is not used , and after looking into it more while Un-linking the Fans in PXOC, I see GPU-Z is actually reporting the GPU Fan Tach as the Main Fanspeed Sensor and the Power Fan ( RH Fan for VRMs etc) is listed seperately as it should be , I was just expecting to see them listed as Left/Right in the Sensor Window. GPU-Z having a Middle Fan Sensor available showing no reading was what confused me , I thought the GPU Fan wasn't being reported but I guess it is there for Cards with the 3 Fan Sensors enabled / used like yours.
HWInfo64 is reporting Both now too, in a second section Under GPU , in the Main section for the GPU it only shows the GPU Fan RPM.
KB.