I gave up long ago on Comcast supplied gear for v4 statics with v6, leave it in place and ignore it
Save yourself the pain !
Simple solution, a pfSense firewall, tunnel IPv6 (6in4) to Hurricane, use the many DNS resolver tricks to strip AAAAs from its (pF) local resolver (dnsmasq) to return only IPv4 A records for certain domains to avoid (Netflix et al) tunnel block issues and I am a happy camper. Plus I avoid
all the junk about static V6's web servers, 6 PTR's etc.
Modems/routers that return busted PD's, DHCP6 that is mis implemented, gways that have VoIP ALG permanently on (you know who). There's no escape from the technical mediocrity of Comcast's engineering of cust nodes. I have great respect for their transit, core and most DOCIS 3 features crew, but the end node device guys can take a flying leap.