[Windows] Why are IPv6 DNS not pushed by our server in some cases?

This issue occurs ONLY under Windows.

Sometimes (the problem is not systematically reproducible) after the VPN connection, inside IPv6 routing table entries you can see routes in identical ranges with different gateways:

>route -6 print
...
 38    259 fde6:7a:7d20:16::/64     On-link
 38    259 fde6:7a:7d20:16::/64     fe80::8
...

When this occurs, DNS6 can't be reached with subsequent delays and issues.

>ping fde6:7a:7d20:16::1
Pinging fde6:7a:7d20:16::1 with 32 bytes of data:
Destination host unreachable.

We think the problem is caused by an OpenVPN bug (at least in 2.4.4), currently under investigation.

Current workaround

Eddie automatically detects the issue and resolves it.

For those who don't use Eddie (and generate .ovpn files with Config Generator), our servers don't push DNS6 directive if Windows (or older versions of Eddie) is detected, until the issue is resolved in OpenVPN code.