Apple Maps doesn't work well with locations that have large zip/ postal codes etc. Why Apple doesn't bookmark the co-ordinates (or even show the user the co-ordinates) is beyond me, but there is a way to workaround it… use Google maps!
The Google URL has the Latitude & Longitude of the centre of the view. e.g.
https://www.google.com/maps/@38.8080894,-116.5795965,3957m/data=!3m1!1e3
Copy the first two figures & search those values in Apple Maps (38.8080894,-116.5795965). You can add that as a contact but Apple Maps will fill in an inaccurate location. Edit that address & paste in the co-ordinates for the address (or add it as a second address).
This should be submitted as a bug if you want Apple to fix this.
http://apple.com/feedback/