There's a few reasons you may be getting a DNS leak while using the OVPN client. One of the more common reasons is that you may be using an application like Avast or AVG; both of these have a feature called Real Site/Secure DNS.
The OVPN client have two settings to prevent DNS leaks. The first one is called Prevent DNS leak and the second one is called DNScrypt. Most of the time, activating both of these will completely fix any DNS issues you may have, even those by Avast and AVG. You can activate both of these settings under Account in the client.
Something else that can affect your DNS addresses if you use Firefox is DNS over HTTPS. For instructions of how to disable that in Firefox, refer to this article: https://www.ovpn.com/en/faq/troubleshooting/dns-leak-when-using-firefox