I just got a Z2 yesterday and I notice the exact same thing. The Z2 dialer does not integrate well with Google Voice. If you make an outgoing call through the dialer and it's routed to Google Voice you see no contact information and the number in history is the Google Voice trunk number assigned to the contact.
I have never seen an Android phone that did not integrate seamlessly with Google Voice so I'm very disapointed with the Z2 in that regards. I'm also disappointed that not a single professional review mentioned this. Had there been mention of issues with Google Voice integration, I doubt that I would have spent the money to buy this phone unlocked.
As it is, I really like just about everything else about the phone and I'm getting tired of the many issues with Google Voice anyways so I'm not sure that it's worth the hassle of returning the phone
FYI, I also had issues with text forwarding to the Z2 until I deleted and reinstalled Google Voice so I'm not sure if that issue will crop back up again because that would be a deal-breaker for me