Do other mDNS services work? What is the firewall being used? mDNS leverages multicast in order to communicate, and most firewalls will block this by default. If the firewall sits between the user vlan and the vlan where the Home Assistant resides, the firewall could be blocking the traffic attempting to flow between these two devices.
From each of your APs, if you use the command "show airgroup servers", do you see the Home Assistant listed as a server on each AP?