I'm not experiencing this with TWC cable modems but I've heard of another user having a similar issue in another TWC market. The dhcp6c client has many options which I wish was changeable via the gui but you can change manually..
maybe these threads can help?
https://forum.pfsense.org/index.php?topic=76322.msg426945#msg426945
https://forum.pfsense.org/index.php?topic=73760.0
dhcp6c.conf man
https://www.freebsd.org/cgi/man.cgi?query=dhcp6c.conf&apropos=0&sektion=0&manpath=FreeBSD+Ports+8.4-RELEASE&arch=default&format=html