been configuring OpenWRT multiple instances and hit some - I guess, undocumented issues:
Instace names must not overlap with interface names. If they do (like instance name guest and interface name guest), they will wipe out each other in in UI and the instance won't work.
The second and following instances must exclude the loopback interface, as it is enabled by default (but can be overseen in the UI). Otherwise dnsmasq fails to start.
Regarding the uniqueness of section names. Mhm yeah that should be more explicit mentioned.
But the "issue" with dnsmasq is documented and well understood default behaviour of dnsmasq.
As with any other Linux distribution the user is expected to also read upstream documentation.
But iirc then regarding DNSmsaq and excluding loopback on additinal instances is documented in the examples, or did you found one in the wiki where it's missing?