To clarify, my comment above, mDNS reflecting does not fix Matter-over-WiFi devices across vlans. See my other post explaining why Matter cannot work with vlans/reflectors by design. The HA Matter server must be on the same subnet (vlan) as the devices and TBRs (at least the current implementation). Personally, I use a second network interface on the IoT vlan which has worked fine for years.
The OTBR server (otbr-agent) only communicates with HA, and even that is very limited. The OTBR server and the Matter add-on server never communicate with each other; Matter server just relays information between HA (the Matter Integration, over websockets) and your Matter devices (which, if they are Thread, will be routed through your OTBR — like a router — but not talk to it directly, if that makes sense).