@darkonc
To my knowledge, this needs to be configured manually in pfSense. The LAN interfaces get their subnets by following the WAN interface, however, the WAN cannot follow itself. In other words: if the prefix change, one needs to manually configure it again - or have some sort of scripting to do this.
All tho I get that "providers should keep prefix fixed, preferably, for the time the contract is active", in reality, most of them do not. They change the prefix if you change/upgrade your router and depending on the configuration you change on the router it can trigger a new prefix (it is based on the DUID in the case of Telus). In my mom's house (Brazil), the IPv6 prefix changes almost every 3 weeks with default configuration (why? to annoy users maybe...).
In short, I agree with everything you said, I would like to have the IPv6 set to the WAN, but I can live without it.
Thanks,
JrBenito.