I've noticed that in EdgeOS 1.9.1 the subnet 'domain-name' option is actually setting 'option domain-search' in dhcpd.conf rather than 'option domain-name'. This is resulting in the domain name not showing up as the DNS domain on any of my DHCP clients.
Not setting 'domain-name' and instead manually setting 'subnet-parameters "option domain-name "lan";"'' results in the correct dhcpd.conf parameter being set and in turn DHCP clients correctly receiving a domain name.
According to https://linux.die.net/man/5/dhcpd.conf, 'domain-search' isn't even a parameter that exists so I assume this change was unintentional?