I am consistently getting DNS lookups that fail consistently across the environment (example below). There does not seem to be consistent timing / behavior for when they file. Based on other issues I ran "pihole repair" and retested but am still seeing consistent failures. This instance has been up and functional for a long time but this behavior seems to have become prevalent after a I switched to an Eero based Wifi system. I have configured Eero with Custom DNS specifying the PiHole IP Address (192.168.0.127).
PS C:\Users\Matt> nslookup discourse.net
DNS request timed out.
timeout was 2 seconds.
Server: UnKnown
Address: 192.168.0.127
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
*** Request to UnKnown timed-out
This is the behavior I am seeing consistently - for some amount of time no resolution will occur and then shortly after it will pick up just fine. Not sure where to look to understand what is happening within PiHole at this time.
It seems obvious in hindsight what the issue was. As much as I tried to disable ipv6 throughout my environment I was unsuccessful. I ended up adding the ipv6 address to my Eero Custom DNS (Primary ipv6) and this resolved my issue.
Commenting here in case anyone else finds themselves in the same situation.