This seems to be a behavior difference which only started in either 18.10 or 19.04.
avahi-resolve --name my.subdomain.local returns a valid IP address and is working it doesn't work outside of that, i.e.
The name resolution does still work though for
subdomain.local. This seems to be related to the fact that
*.subdomain.local is resolved as mDNS CNAME.
passwd: compat systemd group: compat systemd shadow: compat gshadow: files hosts: files mdns4_minimal [NOTFOUND=return] dns networks: files protocols: db files services: db files ethers: db files rpc: db files netgroup: nis
nameserver 127.0.0.53 options edns0