DNS leak tests show my browser is still using my ISP DNS servers

[Bertrand31]

I have AdGuard Home running on my home server, and my router is configured to use it (with a fallback to 9.9.9.9).
AdGuard itself has a couple of upstream options, all from either Quad9 or DNS4EU.

When I run nslookup locally, I can see all DNS calls being resolved by my home server. I can also see all DNS calls from all my devices in the AdGuard query log.

Yet, when I try DNS leak tests (this one, this one or this one), they all tell me I'm really using my ISP DNS servers (which are now nowhere in my configuration, not even as a fallback).

How could that be? Is it something about these online tests that means they can't see my local DNS server? But they're also not seeing my router's fallback to 9.9.9.9, so what are they really testing?

(Before you ask, yes I have disabled DoH in Firefox)

[Bertrand31]

I think I solved it myself: by looking up the whois of the domains, I found out that they're not actually my ISP's, but Quad9/DNS4EU