Actually, there are some apps and even phone level things that do try to call to custom DNS, ignoring all the phone settings, including those defined in the global settings. Termux nslookup is one I can think of at the top of my head that ignores the phone’s settings and instead tries to call to Google DNS. I’ve got DNS default blocked in a custom script for AFWall on my phone, excluding calling my custom DNS, and see the block frequently hit. Just now checking, I see 54 blocks on 8.8.8.8:53, 2 blocks on 1.1.1.1:53, and 16 on “other” port 53 (catch all block).

Think the best solution is either a router firewall setup if always on the wifi, or a phone firewall app that can act as a VPN and just default block everything, or something like that. If rooted, AFWall does wonders.