Are you sure this isn't a case of different matter version support? In which case, in my experience, thread border router works just fine, but the controller needs to support such devices.
Apple: Keeps Thread credentials locked to HomeKit's border routers.
Google: Shares some credentials, but only within Google Account environment.
Amazon: TBD, but their Matter implementation is mostly cloud-tied.
Samsung: Hybrid approach; still best when used inside SmartThings, their 1.4 update seems to support for joining existing Thread networks. Still have to test it.
So, even though Thread theoretically allows full interoperability, no vendor wants to be reduced to a dumb router in someone else’s ecosystem.
there is no easy way to bridge Apple Thread to Home Assistant or Google Thread, even though it is theoretically supposed to be possible from a protocol standpoint.
If you have such solutions, let me know, because I would take full advantage of it, and will regale your contributions in multiple home automation threads.
The Home Assistant iOS app can extract the Thread credentials from Apple’s border routers and send them to Home Assistant. The documentation for what happens thereafter is not amazing.