After a lot of testing, searching, and experimenting with settings in the Windows 10 client, I've not been able to solve a DNS leak in the Windows 10 client. I have, however, been able to stop the leak by using a newer version of OpenVPN.
The Windows AirVPN client currently uses OpenVPN version 2.3.8, a version which does not support the "block-outside-dns" option, which would solve the DNS leak. Version 2.3.9 supports this option, and the current version is 2.3.10, so upgrading the OpenVPN version in the Windows client would solve the leak. More here: https://www.dnsleaktest.com/how-to-fix-a-dns-leak.html
I've tested the option in the most recent version of OpenVPN(Without the AIrVPN client) and the leak is fixed. The latest version is available here: https://openvpn.net/index.php/open-source/downloads.html
Another issue with the client is that when you try to specify a custom path to OpenVPN, it doesn't work and uses the built-in version with the client.
So this is a request to upgrade OpenVPN in the client and enable the "block-outside-dns" option too. Fixing the custom path would be great as well. I think this would help us all because privacy is a very important part of VPNs.