I have been trying for ages to get Govee Matter based bulbs to work on my IoT network, which is a separate VLAN on my Ubiquiti UDM. It was really bizarre; the bulb would connect and it would work in the Apple Home app for ~30 seconds and then go offline, never to return again unless I hard-reset the bulb and re-added it. Whereupon it would work for ~30 seconds and then fail.
Govee Support is about as helpful as you might expect. If it did not fit into a canned answer from their runbook, they were clueless.
I tried using tcpdump on the UDM itself, but that yielded almost nothing. Oddly, it did show some IPv6 mDNS back and forth, but nothing that was really useful.
Then, while switching my phone back and forth from my main network to the IoT network, I noticed that the main network was giving the phone an IPv6 address, where the VLAN was not. So just out of curiosity, I gave the IoT VLAN a made up static IPv6 address with a /64 prefix: ff30::/64. As soon as I added that, and then added the bulb, it stays connected. Every other matter device I have tried from a plethora of other vendors worked with no issues, and I knew v6 packets were allowed, if not explicitly defined, so it did not make any sense that it would be an issue, as devices can self-assign addresses (I assume, anyway; similar to v4), and that is likely how the other devices were working. Govee does not appear to support self-assigned v6 IPs, and so ultimately fails.
Hopefully this will be of help to someone else struggling with Matter devices.