Howdy,
Spent the better part of 5 days, trying to figure out why homekit devices would connect to my hub and then a minute or so later go not responding, tested tons of settings, IGMP snooping, multicast toggling etc. Also I noticed that my ESP32 devices would do the same in HA. Turns out 5.3.21 firmware on my nanohd was blocking it, rolled back to 4.3.21 and it still didnt work, went down to 4.3.20 and voila everything started working again perfectly. Works across vlans (need to have mdns enabled in controller).
It was weird because anything on ethernet would resolve constantly, but wifi would after it first connected but then 15-30 seconds later I would get unknown host (when doing an ICMP) even though I could still ping it via it’s IP. Hopefully this helps someone else down the road.