This change does not apply to you if you use pfSense as an OpenVPN server, but rather when you use it as an OpenVPN client.

Where users are redirecting all of their traffic to a VPN service such as Private Internet Access (PIA) or ExpressVPN.

In some scenarios, users reported back that their DNS was leaking.

You can handle this a number of ways.

One possible solution would be use to DNS over TLS as described in Configuring Quad9 on pfSense.

Another option is to go to Services → DNS Resolver → General Settings and switch the outgoing network interface to LAN only instead of all.

As always, don't forget to click Save after making your changes.