Bill Woodcock

Sorry, sorry, not a Windows user myself, so didn't realize that, but it makes sense. My apologies.

Have you contacted support@quad9.net? It's difficult to know what the problem is without a bit more information. But Quad9 per se will do the same thing regardless of what OS you're running. It's possible that there's an unexpected resolver behavior on your client (the OS using another recursor when Quad9 blocks, for instance), or that your queries to Quad9 are being intercepted along the way. Are you using DNS-over-TLS, DNScrypt, or DNS-over-HTTPs? If not, and you try one of them (easiest might be try to replicate the problem inside Mozilla, using Quad9 over DNS-over-HTTPs), do you also see the intermittent failure to block?