For me this was indeed the solution to bind a lease to a specific instance, thanks!
option instance '<DOMAIN>_dns'
I had to run the following to have the settings being applied ofcourse:
uci commit dhcp
service dnsmasq restart
This post is rather old, but maybe it can help others (adblock per instance): Adblock for specific devices, tutorial to create multiple dnsmasq instances? - #10 by dibdot
I`m running AdGuardHome as an ad blocker service. I defined rules per subnet so every instance has its own set of block lists/rules and different upstream DNS servers. Working like a charm!