Just to close this out... To avoid any issues at the next upgrade I backed out the two changes in the patch and uninstalled the dhcpleases6 package.

Then I added a new gateway on my external router that points to the hyper-v router's ipv6 address on its WAN side (the external router's LAN side), and added a static route to direct the delegated prefix subnet to that gateway. This configuration also works to allow IPv6 to work from the internal hyper-v router's LAN side. These two configuration items will be easy to back out at upgrade, assuming the issue is addressed in the next release.