So that means if you have a v4 only VPN provider all IPv6 happily goes over the default route.
This is not surprising to me at all, traffic should follow the default route that is given. If you are privacy conscious you should already know how to disable IPv6... Honestly if it is a bug anywhere it is a bug in the VPN providers that they are not providing IPv6 services for their customers.
Thankfully with OpenVPN IPv6 setup is simple, and while it doesn't provide the privacy extensions like SLAAC (you can only get a single static IPv6 address on the other end of the tunnel), it does allow you to easily tunnel IPv6 traffic as well. I personally do this by pushing 2000::/3 across from my OpenVPN server.
Not really. Since VPNs have many different purposes (privacy is only one, and arguably not even the most common one), disabling all IPv6 traffic whenever a v4-only VPN is enabled may be a bad default for other use cases. In fact, I would be rather surprised if they did this (though pleasantly surprised).