Pretty sure all of the AG DNS servers are experiencing peak load capacity, and have been for a while now. I've noticed at random that the AG DNS servers will stop resolving for a minute or two while giving DNS errors in the web browser. There's supposed to be server-side updates and upgrades landing at some point, hopefully soon as to stabilize the load on the AG DNS servers. It's needed for the new AG DNS service that's launching at some point this year.
Until the server-side updates/upgrades have been pushed out I've temporarily switched to Cloudflare and Google's DNS as my upstreams.