I was wondering about that many (in relation) unknown/empty DNS requests in the stats and made a quick research.
I use “dnscrypt-proxy” as resolver using DoH to Cloudflare.
Based on the CF stats and the logs from dnscrypt, every 15 minutes an “empty” request is made.
The tool makes a health and performance check every 15 minutes ( not documented) but without dns request, only HTTPS. To disable this functionality go into your config and set
lb_estimator = false
maybe in other tools you have the same behavior.