No, I don’t use secure DNS in my browser, I only use the VPN itself that is connected by Wireguard using RethinkDNS
Checking the site you recommended to me there is no DNS leak, so it seems to me the only site that claims to exist leaks is Mullvad’s own but it doesn’t happen every time. Very strange indeed.
Do you think this whole situation could be a false positive of DNS leakage by the Mullvad website?