pfSense

IPv6 NPt on its own works, and IPv6 policy routing on its own works, but if traffic hits a rule that sets it on a path that would do both, the traffic never exits the firewall.

Example (made up addresses -- real ones available upon request):
NPt on WAN for internal xxxx:yyyy:2::0/64 to external wwww:zzzz:2::0/64
WAN_GW6 is wwww:zzzz:1::1

Example Rule on LAN:

WAN_GW6 = " route-to ( gif0 wwww:zzzz:1::1 ) " 
pass  in  quick  on $LAN $WAN_GW6 inet6 from any to any keep state

With the gateway on the rule, traffic enters and a state is present on the LAN interface only

igb0_vlan40 ipv6-icmp 2001:470:1f11:ssss:ssss:ssss:ssss:ssss[61473] <- xxxx:yyyy:2::7[61473]       NO_TRAFFIC:NO_TRAFFIC
   age 00:00:06, expires in 00:00:10, 7:0 pkts, 392:0 bytes, rule 290
   id: 0100000058471e69 creatorid: 285c14c6

Without the gateway on the rule, traffic enters and exits and NPt is shown on the exiting state

igb0_vlan40 ipv6-icmp 2001:470:1f11:ssss:ssss:ssss:ssss:ssss[19877] <- xxxx:yyyy:2::7[19877]       NO_TRAFFIC:NO_TRAFFIC
   age 00:00:04, expires in 00:00:07, 2:2 pkts, 112:112 bytes, rule 290
   id: 0100000058471da9 creatorid: d4262046
gif0 ipv6-icmp wwww:zzzz:2::7[19877] (xxxx:yyyy:2::7[19877]) -> 2001:470:1f11:ssss:ssss:ssss:ssss:ssss[19877]       NO_TRAFFIC:NO_TRAFFIC
   age 00:00:04, expires in 00:00:07, 2:2 pkts, 112:112 bytes, rule 117
   id: 0100000058471daa creatorid: d4262046

The combination worked without issue on 2.3.x and earlier versions.