@GuyMan's response sums it up. This topic also came up a while ago and there is some good information at Is it finally time to embrace Matter? < Hint NOT! - #73 by jvm33. Basically, start at that post and scroll down.
I haven't had a chance to confirm Thread 1.4, but last time I used the Discovery app my 8 devices (2 Apple TVs, 3 Hub Maxes, and 3 Nest WiFi Pros) all were using the same Thread Network name (coincidentally it was created by Nest, but not surprisingly since I had them longer than the Apple TVs). The big thing at the time was everything being setup via an iPhone. Google devices via the Google Home app can share Thread credentials with iOS and keychain. I'm pretty sure that is the key right now.
Unless someone knows otherwise, I do not think the tools really exist to identify how this is happening with Thread. It's kind of a black box when it comes to troubleshooting. The one thing we do know is that having two separate thread networks greatly impacts battery devices in a negative manner. Basically, it causes faster battery drain since the device has to send multiple messages. Mains powered devices do not suffer from being connected to multiple controllers.
