Does that setup fall back to 8.8.8.8 if 9.9.9.9 fails to resolve?
Quad9 has a very aggressive blocking policy (my site with user-uploaded content was banned without even reporting the malicious content; if you're a big brand name it seems to be fine to have user-uploaded content though) which this would be a possible workaround for, but it may not take an nxdomain response as a resolver failure
