Basically – If you are using OpenVPN with IPv6 support, MacOS will not recognize it and only allow IPv4 DNS resolution.

References:
https://openvpn.net/vpn-server-resources/limited-ipv6-support-built-into-the-access-server/
https://apple.stackexchange.com/questions/304215/how-to-add-aaaa-flag-ipv6-to-dns-resolver-configuration-on-sierra
https://www.tunnelblick.net/cUsingScripts.html
https://tunnelblick.net/cConfigT.html#creating-and-installing-a-tunnelblick-vpn-configuration