A restored iPhone can carry another phone’s Home Assistant registration and sign-in, causing both phones to report as one device and letting a sign-out on one break the other. The Companion app detects that copied registration and asks whether the restored phone should become a new device or replace the old one.
Setting up a new device signs in again and creates an independent registration while leaving the original phone alone. Replacing the old device explicitly takes over its registration. Until the choice is settled, the restored phone holds back location and sensor reports that would otherwise overwrite the other phone’s readings.