This is almost certainly a networking problem. Commissioned Thread devices use a dedicated IPv6 subnet and your HA server seems to have a problem finding or accessing that subnet. Read the pinned helpful hints and let us know if you have any unusual network topologies (multiple vlans, dual-homed servers, etc) that could be complicating things.
BTW there is no need to add the Apple TV or share credentials or make preferred if your Matter devices are already commissioned to Apple Home. You just need to get the add-on and confirm the Thread subnet is reachable, then you can share your Apple Home Matter devices with HA.