Automatic on Android always falls back to Google or Cloudflare DNS in the same way systemd DNS resolving works. Or if that does not work, ISP is being sent whatever domain queries user is requesting directly. I am going off from connecting the dots between what I know about Private DNS from Android documentation, and what the Reddit poster did not mention who Mullvad cited in their blog post. I am assuming that the time gap between Android's killswitch turning off and on with always-on settings is giving time for DNS queries to go through (detected by Wireshark), and since the default DNS provider is almost never set by people on Android, this may be happening.