@theipw interesting. I'm just using Android mobile phone connected to a hotspot on Android mobile.
I left my phone for a while just now and upon returning had a similar issue even with Q9 disabled. It cleared much quicker but it makes me wonder if Q9 is the real culprit.
I shall continue without for a while.
@happyborg so I have a similar problem using technitium DNS server and the quad9 DoH endpoint.
I frequently see ServerFailure spikes that obviously go down because of caching but it's not reliable.
for now my im using dnsforge.net which doesn't fully solve the problem but behaves a lot better.