Apple - Alternate DNS resolution for Safari on macOS?

It sounds like you've done a pretty good job of covering all of your bases... as far as IPv4 is concerned. Maybe it's IPv6 that's causing you headaches? I believe that disabling IPv6 on the connection will fix the problem. IIRC IPv6 can autoconfigure itself without DHCP. If the rest of your network works with IPv6 then the queries could still be working over IPv6.