That might explain the problem. Assuming adguard returns an nxdomain for blocked sites then the devices will try with their secondary DNS server and get to the blocked site

Sorry I misread your post. You shouldn’t need the 172 address in your DNS config, stick to the 127 address only.

In regard to the issue itself. Do your devices show their DNS server as the adguard server?

Your adguard config looks strange. The examples shown list different DNS providers but you have pointed it back at itself for its DNS. I don’t understand why you would do that.

I use Arch because it makes installing almost any software package trivially easy via the AUR and if you run into issues, the wiki is there to help.