i don't use uBlock Origin but can't confirm any leak.

Did you try with clean browser profile or with another browser?

I use Firefox with the Apple profile on macOS, uBlock Origin installed and running and all tests show only NextDNS. I run them on other browsers and it also only shows NextDNS, but just to confirm the uBlock behavior.

If you use google product then it's normal because all google products have embeded google dns inside them. So it's a random choice by the apps for any available known dns server. If you want to avoid this just tick the option that sounds like "force router dns redirection" in your router if it's supported, it will redirect all dns request port 53 to the router only.

I was seeing this - eventually I found it was caused by having the NextDNS log screen open & refreshing as the test was completing.

It looks like there is something that resolves the domain DNS in order to visit the page & fetch the favicon on the log page.

Did you ever get this sorted? I seem to have it with running Pihole but instead of Google, I have Cloudflare popping up. Asus router as well.