Thanks. The filter has the same problem. It doesn't automatically track any updated Prefix Delegation assignmant. What I have now is this in /etc/rc.local:
# Finally reject the unmapped part of our PD allocation. Needed to
# prevent a routing loop for our unmapped PD addresses. We don't want
# to send them upstream because the router will send them right back.
#
# we should really do this automatically.
/sbin/ip route add unreach 2601:x:x:x0::/60
It works well now, but I'm sure I'm going to forget to update it if my PD changes.