router99 If you're using a VPN, it's recommended against having a Private DNS server configured. If you want to filter traffic while using a VPN, use a VPN service app able to do both such as RethinkDNS. Private DNS also interacts strangely with multiple profiles since each profile has their own VPN configuration but Private DNS is global. Either leave Private DNS on the default Automatic mode or set it to disabled when using VPNs.
Apps and web sites can detect the configured DNS servers by generating random subdomains resolved by querying their authoritative DNS server. This can be used as part of fingerprinting users. If you're using a VPN, you should consider using the standard DNS service provided by the VPN service to avoid standing out from other users.