How to debug DHCP client not using remote DNS server for hostname lookup on both Linux & Windows

Question

I'm having some trouble setting up a DHCP/DNS solution - specifically, getting the clients to use the DNS service provided by the router.

I have an "EdgeRouter" that I have set up as a switch (interface switch0), with a DHCP server, as well as a Dnsmasq service to provide resolution of DHCP hostnames as per this guide

# show service dhcp-server
 disabled false
 hostfile-update disable
 shared-network-name my-dhcp {
     authoritative disable
     subnet 192.168.7.0/24 {
         default-router 192.168.7.1
         dns-server 192.168.7.1
         domain-name local
         lease 86400
         start 192.168.7.100 {
             stop 192.168.7.240
         }
     }
 }
 static-arp disable
 use-dnsmasq enable

I have two clients - an Ubuntu 18.04 linux laptop (call it ubuntu), and a Windows 10 laptop (call it windows), both of which connect to the switch and get an IP address via DHCP. I can fully ping between each client and the switch management interface, and between each other. I've also examined the DHCP records received by each client and verified that they receive a DNS server address of 192.168.7.1, which is correct.

On the switch I can confirm these leases with:

$ cat /var/run/dnsmasq-dhcp.leases
1420222142 ec:8e:b5:94:bd:8f 192.168.7.175 ubuntu *
1420219707 c8:d3:ff:6f:42:15 192.168.7.100 windows 01:c8:d3:ff:6f:42:15

On ubuntu I can also verify that the DNS server on the switch is working correctly:

$ dig @192.168.7.1 ubuntu

; <<>> DiG 9.11.3-1ubuntu1.13-Ubuntu <<>> @192.168.7.1 ubuntu
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15747
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;ubuntu.            IN  A

;; ANSWER SECTION:
ubuntu.     43200   IN  A   192.168.7.175

;; Query time: 0 msec
;; SERVER: 192.168.7.1#53(192.168.7.1)
;; WHEN: Tue Nov 03 12:21:15 NZDT 2020
;; MSG SIZE  rcvd: 53

Similar response for a dig with windows.

If I ssh into the router I can ping ubuntu or ping windows with no problem - the name resolves.

The problem is that neither the Linux nor Windows laptop seem to resolve when I ping from Windows:

> ping ubuntu
Ping request could not find host ubuntu. Please check the name and try again.

Or from Linux:

$ ping windows
ping: windows: Name or service not known

Also from the Linux client:

$ nslookup windows
Server:     127.0.0.53
Address:    127.0.0.53#53

** server can't find windows: SERVFAIL

From what I understand, my Linux laptop is running its own dnsmasq and /etc/resolv.conf looks like this:

nameserver 127.0.0.53
options edns0
search local

I'm not sure how to check the dnsmasq status to see if it's forwarding to/from the DNS server address that DHCP obtained.

On the Windows side I have no idea how to debug this.

Also, bizarrely, just after writing this, the Linux side of things suddenly started working:

$ nslookup windows
Server:     127.0.0.53
Address:    127.0.0.53#53

Non-authoritative answer:
Name:   windows.local
Address: 192.168.7.100

I didn't change anything! Maybe something figured itself out over a period of 10-20 minutes. But the Windows side is not working.

What would be the best next step for figuring this out?