Just had a weird issue setting up Wireguard and OpenVPN servers on OPNsense

I created dual IPv4/IPv6 VPN's which become the default Internet Gateway for Road Warrier clients.

But I noticed a problem when checking the connection with whatismyipaddress.com IPv4 would be detected but IPv6 would display Not detected

If I used curl I would get the expected Gateway IP's

curl -4 ifconfig.me; echo
52.147.37.48

curl -6 ifconfig.me; echo
2603:1010:200::32c

I ended up tracking it down to an MSS problem. After fixing it IPv6 would reliably be detected:

Go to Firewall => Settings => Normalization and add a rule for each interface OpenVPN / Wireguard and set the MSS to 1340