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 automatically attempts to prevent DNS leaks by checking the DNS settings on the adapters every second to ensure no installed software attempts to change the used DNS servers
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