IPv6 Default Route is "static" Instead of "ra", IPv6 Eventually Stops Working

[Kenneth R. Crudup]

I have NM (1.2.6-0ubuntu0.16.04.2) on my eth0 interface. The IPv6 setup is "Automatic" (via the NM GUI). When first starting up (or if I manually run "nmcli connection up Ethernet") I have IPv6 connectivity, as NM will hear my router's RA and assign a default route via the router's advertised link-local address:

default via fe80::4cf4:93ff:fec3:321 dev eth0 proto static metric 100 pref medium

The problem comes some time later when my router (Google WiFi) for some reason will change its link-local address, and by extension, the IPv6 default route. I don't know why this happens, but I do know the router will send that new LL default address in response to Router Solicitations (which this box generates, BTW), and which I've verified via "traceroute"- yet NM will ignore the new IPv6 default route and continue to use whatever value it got when booting (or from "nmcli connection up").

At this point I have no external IPv6 connectivity.

Even if I delete that default route and manually re-assign it (and even using the "ra" vs. "static" designation) when the router sends another RA NM replaces the default route, and with the OLD bad address.

Please fix NM so that it pays attention to the RAs that routers send.

Apparently I can't manually assign a default route (which can also be "<my ISP's assigned/64>::") and also have "Automatic" or "Automatic, Addresses Only" for the address selection (I need DHCPv6 to run, I don't want to manually assign an IPv6 address). Trying to add "gateway=" to the [ipv6] config section of /etc/NetworkManager/system-connections/Ethernet is ignored.