Cannot identify network devices (clients)

When I access the logs via the NextDNS webpage (https://my.nextdns.io/xxxxxx/logs) I no longer see individual devices and all queries are listed either against my IPv4 address or the IPv6 address provided by my ISP (rather than the individual temporary or delegated IPv6 addresses that I would expect).

The issue started 3 days ago but I don't think I changed any setting of significance recently (famous last words...).

Until the 7th of this months I could view devices making the requests:

Now all requests can be found in the Unidentified Devices bucket:

I am surprised to see the full IPv6 (feed) WAN address provided by my ISP, rather than the delegation derived IPv6 temporary addresses used by clients. If I check my external IPv6 addresses (eg via https://whatismyipaddress.com) it does display the correct individual temporary IPv6 address for the client used:

I use pfSense on a Netgate appliance.  I'm relatively new to pfSense (previous an EdgeRouter CLI user) so there is a chance I have borked a setting somewhere.  I use the NextDNS configuration that you paste into the resolver Custom Options field with no other customisations.

What am I missing?