clog /var/log/resolver.log | grep "t assign requested address for" | tail
Mar 15 12:12:11 pfSense unbound: [23988:0] error: can't bind socket: Can't assign requested address for fe80::224:9bff:fe0b:4a75
Mar 15 12:12:12 pfSense unbound: [23988:0] error: can't bind socket: Can't assign requested address for fe80::211:6bff:fe34:b286

All the errors mention only two IPv6 addresses and they correspond my 2 WAN interfaces:

clog /var/log/resolver.log|tail -1000 | grep "Mar .*t assign requested address for" | sed -e 's/^.*for //' | sort | uniq -c
 526 fe80::211:6bff:fe34:b286
 474 fe80::224:9bff:fe0b:4a75

ifconfig | grep -B3 'fe80::211:6bff:fe34:b286\|fe80::224:9bff:fe0b:4a75'
re0: flags=8843 <up,broadcast,running,simplex,multicast>metric 0 mtu 1500
	options=8209b <rxcsum,txcsum,vlan_mtu,vlan_hwtagging,vlan_hwcsum,wol_magic,linkstate>ether 00:11:6b:34:b2:86
	inet6 fe80::211:6bff:fe34:b286%re0 prefixlen 64 scopeid 0x2 
--
ue0: flags=8843 <up,broadcast,running,simplex,multicast>metric 0 mtu 1500
	options=8000b <rxcsum,txcsum,vlan_mtu,linkstate>ether 00:24:9b:0b:4a:75
	inet6 fe80::224:9bff:fe0b:4a75%ue0 prefixlen 64 scopeid 0x7</rxcsum,txcsum,vlan_mtu,linkstate></up,broadcast,running,simplex,multicast></rxcsum,txcsum,vlan_mtu,vlan_hwtagging,vlan_hwcsum,wol_magic,linkstate></up,broadcast,running,simplex,multicast> 

So there's no such error for the IPv6 addr of my LAN interface and there are too many for my 2 WAN interfaces.

I'll admit I have little knowledge on IPv6 but I don't (intentionally) use it anywhere in my LAN.

Googling around I found that (counter-intuitively)  many were suggesting to leave the DNS resolver bind to ALL interfaces. But this was set like this by default. I've tried binding DNS resolver to my LAN only (not even LAN IPv6) and then restarting the daemon but the errors kept coming.