I think the problem is Reddit user (who Mullvad cites) not knowing that the Private DNS feature in AOSP/Android defaults to Google or Cloudflare DNS, and that you need to set a custom DNS of your choice to prevent this.
AdGuard provides a whole list of DNS providers to pick from. Pick a hostname from DNS-over-tls row for any provider, remove the "tls://" part and enter the rest in Private DNS custom option.