Just to make it clear: For me, since the RPB1 all up to the current version 3.29 of Google Contacts do actually show contact information coming from DAVx5. But they seem to be read-only for Google Contacts.
I neither can select DAVx5 as the default account for new contacts, nor can I edit any of the contact-information coming from DAVx5.
For pure DAVx5-contacts (without any merged information from the same contact in my Google-account), there’s not even an “edit contact” button on the lower right.
The problem doesn’t exist for other CardDAV-clients like e.g. CardDAV-sync free, which just works fine and is currently my saviour and daily driver.