WAN IPv6 range can't reach LAN IPv6 range

Yesterday morning, my Comcast service was upgraded and it seems there as a Comcast modem update as part of that process as well.

The Comcast modem delegates an IPv6 prefix for my LAN to my router. This seems to still work. However, I have a device on the IPv6 WAN. What I found is that this WAN device was no longer able to reach my LAN.

Long story short, IPv6 traffic *from* the LAN reaches the device on my WAN. But the device on my WAN was unable to reach the LAN. The device on my LAN isn't being given a valid route from the Comcast modem to my LAN. Instead, the device sends out  neighbor solicitations for v6 addresses that are on my LAN even though the LAN and WAN have different prefixes (so probably the Comcast modem is telling the device on the WAN that LAN IP's are on the WAN too for some reason?). I was able to work around this by switching the device on my WAN from obtaining its address automatically to using a static configuration, where the the route to my LAN's prefix is static. The default IPv6 route is still obtained automatically for this device on the WAN.

Presumably this is still possible to do using SLAAC, but I haven't figure out how, and a static address is completely acceptable for my use case.

I set the IPv6 portion of /etc/network/interfaces (Debian/Ubuntu) to the following:
iface eth0 inet6 static
       address xxxx:yyyy:zzzz:1000::blah
       netmask 64
       dad-attempts    90
       dad-interval    1
       # Routing for the LAN prefix. Will need to add more pairs if I ever want more prefixes. Ugh.
       up route -A inet6 add xxxx:yyyy:zzzz:10e0::/64 gw xxxx:yyyy:zzzz:1000::router
       down route -A inet6 del xxxx:yyyy:zzzz:10e0::/64 gw xxxx:yyyy:zzzz:1000::router

Does anyone know if this is fixable on the modem? I assume it's not.

Does anyone know if I purchase my own Comcast compatible modem if the general IPv6 experience just works better in general? And/or if I would have more control over the IPv6 delegation and its associated routing to my downstream router?