said by voiptalk:
It may be a SWAG, but what makes sense is ....
The /60 that initially caused the problem was not being properly inserted into the CMTS and traffic was unable to route back to you.
Changing the MAC also changed the DUID. That gave you a new PD assignment that was routing properly.
When you changed it back, the old DUID re-appeared and you got the bad addressing back.
So, it's not really the MAC address, per se. But, the DUID value that is derived from it.
True enough with regards to what happened after I cloned a different MAC address into the DIR655's WAN, but the problem occurred with absolutely no changes to the DIR655 on my end. All I did was to power cycle the DIR655 (something that I have literally done 100's of times previously with no problems getting a usable IPv6 connection). There was no reason for the DUID to to have changed (and looking at the saved logs, I don't see where it changed until I cloned the MAC address), or for Comcast to arbitrarily change the PD prefix (or its allocation length).
Here are syslog excerpts showing the last refresh for the PD prefix 2601:5:c80:90::/64 assignment, the first occurrence of the PD prefix 2601:5:c80:a0::/60 assignment, and the assignment of the PD prefix 2601:5:c80:84::/64 for your enjoyment (or for anyone else who is curious):
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