I use pfSense here and have contributed various PPP and IPv6 fixes to pfSense.
I suspect you've got one key element of your configuration missing, which I will come to below.
I would set the "IPv6 Configuration Type" on your WAN interface to DHCP6, which in pfSense (at least as the code currently stands) means DHCPv6 and/or SLAAC. This means that you'll support both variants of WAN configuration listed in TR-187, though Zen only currently support SLAAC. This is a matter of robustness; it's best to accept whatever autoconfiguration Zen offers.
Whilst you're in the WAN interface, if you have a jumbo capable NIC on the WAN port and jumbo capable FTTx hardware (including either flavour of Openreach FTTP ONT and either flavour of Openreach FTTC modem), pfSense supports MTU 1500 over PPPoE using RFC 4638 from pfSense 2.3 onwards - just set 1500 as the WAN MTU. If this causes problems, clear the MTU field which will reset the MTU to the PPPoE default of 1492.
You can test whether the 1500 byte WAN MTU is working using Diagnostics->Command Prompt. Enter (without the quotes) "ping -D -s 1472 -c 4 forum.pfsense.org" in the "Execute Shell Command" box and press Execute. You should get some responses if you have MTU 1500 working. If IPv6 is working, you can additionally try "ping6 -D -s 1452 -c 4 forum.pfsense.org".
You then need to go into Firewall -> Virtual IPs. Create an "IP Alias" VIP on your WAN interface of <your /64 prefix>::1/128 - that allocates an additional address on your WAN port to go alongside the SLAAC address (N.B. it is generally regarded as best practice to allocate additional IPv6 addresses within an existing prefix on the interface as /128 - if you were configuring this address as the only address on your WAN interface statically, you would use /64). In addition, create an "Other" VIP on your WAN interface of <your /48 prefix>::/48 - this is the essential step that you are likely missing.
If you have an IPv4 netblock, you need an "Other" VIP of <lowest IPv4 address>/<allocated width> on the WAN interface - then you can use the additional addresses on your local networks using any combination of routed IP and NAT that you desire.
Let me know how you get on.