Why is my Android phone querying DNS for "nonexistent"?

I have a Google Pixel 3 running Android 11 with the “1.1.1.1: Faster & Safer Internet (Beta)” app installed, version “6.8 (2002)” of Jun 28 2021.
In Settings I have selected WARP+, specified no trusted networks, selected “Block Malware”,
The app is not active on my phone on my home Wi-Fi network. The phone’s Private DNS Mode is Off.
My home gateway is a Ubiquiti ER-6P running v2.0.9-hotfix.2 with dnsmasq 2.85, and serves DNS for the LAN. It forwards queries to your “1.1.1.1 for Families” service.

My phone periodically issues DNS queries for both “nonexistent.cloudflare.com.” and “nonexistent.cloudflare.com”.
When my router sees a query lacking a terminal dot (as in that second name) it appends my home domain.

Here are some sample episodes from my router’s syslog:

Jul 21 21:35:33 gw dnsmasq[545]: query[A] nonexistent.cloudflare.com from 2600:6c55:abc::def
Jul 21 21:35:33 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com to 2606:4700:4700::1002
Jul 21 21:35:33 gw dnsmasq[545]: query[AAAA] nonexistent.cloudflare.com from 2600:6c55:abc::def
Jul 21 21:35:33 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com to 2606:4700:4700::1002
Jul 21 21:35:33 gw dnsmasq[545]: reply nonexistent.cloudflare.com is NODATA-IPv6
Jul 21 21:35:33 gw dnsmasq[545]: reply nonexistent.cloudflare.com is NODATA-IPv4
Jul 21 21:35:33 gw dnsmasq[545]: query[AAAA] nonexistent.cloudflare.com.mydom.ain from 2600:6c55:abc::def
Jul 21 21:35:33 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com.mydom.ain to 2606:4700:4700::1002
Jul 21 21:35:33 gw dnsmasq[545]: query[A] nonexistent.cloudflare.com.mydom.ain from 2600:6c55:abc::def
Jul 21 21:35:33 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com.mydom.ain to 2606:4700:4700::1002
Jul 21 21:35:33 gw dnsmasq[545]: reply nonexistent.cloudflare.com.mydom.ain is NXDOMAIN
Jul 21 21:35:33 gw dnsmasq[545]: reply nonexistent.cloudflare.com.mydom.ain is NXDOMAIN

Jul 21 21:39:42 gw dnsmasq[545]: query[AAAA] nonexistent.cloudflare.com from 2600:6c55:abc::def
Jul 21 21:39:42 gw dnsmasq[545]: cached nonexistent.cloudflare.com is NODATA-IPv6
Jul 21 21:39:42 gw dnsmasq[545]: query[A] nonexistent.cloudflare.com from 2600:6c55:abc::def
Jul 21 21:39:42 gw dnsmasq[545]: cached nonexistent.cloudflare.com is NODATA-IPv4

Jul 21 21:50:47 gw dnsmasq[545]: query[AAAA] nonexistent.cloudflare.com from 2600:6c55:abc::def
Jul 21 21:50:47 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com to 2606:4700:4700::1002
Jul 21 21:50:47 gw dnsmasq[545]: query[A] nonexistent.cloudflare.com from 2600:6c55:abc::def
Jul 21 21:50:47 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com to 2606:4700:4700::1002
Jul 21 21:50:47 gw dnsmasq[545]: reply nonexistent.cloudflare.com is NODATA-IPv6
Jul 21 21:50:47 gw dnsmasq[545]: reply nonexistent.cloudflare.com is NODATA-IPv4
Jul 21 21:50:47 gw dnsmasq[545]: query[AAAA] nonexistent.cloudflare.com.mydom.ain from 2600:6c55:abc::def
Jul 21 21:50:47 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com.mydom.ain to 2606:4700:4700::1002
Jul 21 21:50:47 gw dnsmasq[545]: query[A] nonexistent.cloudflare.com.mydom.ain from 2600:6c55:abc::def
Jul 21 21:50:47 gw dnsmasq[545]: forwarded nonexistent.cloudflare.com.mydom.ain to 2606:4700:4700::1002
Jul 21 21:50:47 gw dnsmasq[545]: reply nonexistent.cloudflare.com.mydom.ain is NXDOMAIN
Jul 21 21:50:47 gw dnsmasq[545]: reply nonexistent.cloudflare.com.mydom.ain is NXDOMAIN

Can you help me understand why my phone is trying to resolve those names?

Thanks!