Thread network Apple TV "keeps trying to join"

Configuration Matter/Thread
1

I’ve got a Thread network set up in HA using a ZBT-2 as the Border Router.
It shows up in the Thread section of the companion app as the only network, with the ZBT as the only Border Router.

I’ve also got an Apple TV, which as soon as I apply power seems to mess things up. Multiple Thread devices drop off line, mostly affecting the devices which are close to the Apple TV.
If I look in the Thread tab of the Matter Server app, I can see that the affected devices or their neighbours are showing the Apple TV as the “external” connection in the list of neighbours.

If I sit and watch the the preferred network box in the Thread section, the Apple TV will pop up as a Border Router for a few seconds every minute or two. It shows up, then goes away again.

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I think that the Apple TV is continually trying to join the preferred network, devices are re-routing to utilise it, then re-routing again when it disappears.

How do I fix this?

Can I either get the Apple TV to actually join the HA Thread network as a Border Router and stay connected, or stop it ever trying to join the network?

2

OK, I think I’ve fixed it.

I deleted any “Thread” references in the Apple Keychain, then factory reset the Apple TV.
I also deleted the Credentials on my iPhone’s companion app.

The Apple TV automatically created a new Thread network, but it shows up as a completely separate network in HA. I just need to avoid using my iPhone to add new devices.
Been up and running for 2 hours, no problems