Naughty IoT ("Internet of Things") devices
I recently discovered that my Samsung TV ignores the DNS server specified by my DHCP server and just uses 8.8.8.8. No wonder I still see ads on it. I had to set a NAT rule to override outbound port 53 and redirect it back to NextDNS.
It seems the only other devices on my network that do so are a couple of work-managed laptops.