After converting from ISC to DNSMASQ I had 1 device that would not work, an Echo Spot device. All my other Amazon devices worked, just not this model. After playing around with just about everything I could think of, I finally resolved it by adding a file on the config.d folder to set "dhcp-broadcast".
I am not a network guy, so I'm badly parroting my google searches and AI explanations, but it seems that this device and some mediatek chips don't like a unicast response DHCPACK. It will only accept it if it was broadcast. From my understanding is the ISC broadcasts but DNSMASQ by default unicasts it.
Once I set this, the device worked. I was also able to get it to work if I created a "Host" entry, but I didn't want to set the hostname, ip address, and all the other things as a long term solution because while I only had 1 device now, if I added other devices, I didn't want to potentially have to add others.
I didn't see an option in the UI to toggle "dhcp-broadcast", but I think an option would be a good addition.