If you want the specific distance from the sensor's plane till the point of focus (what means subject anyway?) you will not get it, simply because it isn't recorded in your file - you can look with ExifTool to see that it says 'Unknown'.
For your sake I looked at a bunch of files generated via different top-notch lenses (Canon L, Sigma Art series etc.) on a flagship body (Canon 1DX Mk II) and it isn't recorded. Technically speaking it is quite tricky to get it with accuracy (focus breathing, distortions etc.), most probably that's why they abandoned it.
It isn't XnView's fault.
The only thing you can do is to display your Focal Length and your F-Number and use a DOF calcualtor to get your DOF (ok, you have a ruler, right?