While experimenting with ProtonVPN's Wireguard configs, I realized that my real IPv6 address was leaking while IPv4 was correctly going through the tunnel. How do I prevent this from happening?
I've already tried adding ::/0 to the AllowedIPs option and IPv6 is listed as disabled in the NetworkManager profile.
Wireguard easily supports dual stack configuration on a single interface, but the VPN server must also have IPv6 enabled. I use AirVPN and I get both IPv6 and IPv4 with a single wireguard tunnel. In addition to the ::/0 route you also need a static IPv6 address for the wireguard interface. This address must be provided to you by ProtonVPN.
If that's not possible, the only solution is to entirely disable IPv6.