An IPv6 leak can reveal your real IPv6 address when accessing an IPv6 resource past the VPN tunnel.
An IPv6 leak reveals your real IPv6 address if:
1) You are connected to a VPN service or self-configured VPN server without IPv6 leak protection;
2) Your ISP supports IPv6;
3) The server you are accessing supports IPv6;
A significant proportion of VPN services and self-configured VPN servers lack IPv6 leak protection.
Red Shiled VPN apps protect against IPv6 leaks — all IPv6 traffic is blocked when the VPN is connected.
You can check for an IPv6 leak at ipv6-test.com.