Does HomePod mini's thread border router work offline (no internet connection)?

I ask this because anytime I have an internet outage, my thread devices go "Unavailable". Once the outage is restored, the devices become available again. I can easily reproduce this behavior. The devices I have are door locks and use Matter over Thread. They are all within a few feet of a HomePod mini and work fine when the Internet is available. But as I said, as soon as the Internet goes down the devices are unavailable. Is this by design or do I have some kind of defective thread border router in my HomePod mini? My understanding is that a thread mesh network is local and should be maintained without Internet connectivity. My locks do not require Internet connectivity.