@wintermute1912
If traffic passes through the tun interface it's in the tunnel, so even if you want to reach 3rd parties DNS servers, the queries and their replies are tunneled, it's not a DNS leak. Even worse: in this way you will never find DNS leaks, even if they are really occurring.
To verify effectively you need to check traffic from the physical network interface. Unencrypted DNS queries from the physical network interface, if not blocked by the firewall, hint to DNS leaks for real.
Kind regards