No. You were probably not leaking. It just explains why Windows was aware of the traffic. Also, doing without a layer of NAT is a bit more efficient.
EDIT:
With a bridged connection the IPv6 address that the router gave your VM is probably "2001:....". This will be within the IPv6 subnet your ISP assigned to the router. Programs running on the VM can see this, and could report it to some remote location. But if you use NAT for IPv6 from the VM, then the IPv6 address a program on the VM can see is a "private" address, and cannot be used to locate you.
I don't know about Hyper-V, but the NAT for VirtualBox needs some extra (rather complicated) configuration in order for NAT to be done for IPv6. Without this the VM would not even have access to IPv6. Which may not be good.