FYI - DHCP-PD is now working better in RouterOS 6.5
Prefix length hints are now available (CLI) only.
/ipv6 dhcp-client add add-default-route=yes interface=<wan interface> pool-name=dhcp-pd \ prefix-hint=::/60
I'd like to encourage people to use prefix-hint=::/48. The router should accept the /60 and deal with it, but it's better to have Comcast's logs show that you requested a proper full-size prefix. I'm almost afraid to ask about the phrase "add-default-route=yes" in the dhcp-client configuration. That seems wrong on the face of it since you should be getting your routing information from RA and not DHCP.
In the case of Comcast (and anecdotally ISC DHCP) - You'll either need to wait out the the lease time (4 days) or ask Comcast to nicely clear out your /64 lease manually. Release/renew doesn't release your current DHCP lease. I was getting A /64 and /60 (/64 had a preference of 255) before Comcast removed the /64 lease manually.
Is it somehow harmful to have both? Owen