The above problem does not seem to occur when the SIM is unlocked, only when the SIM is using a PIN.
A SIM using authentication and no PIN works fine.
I have previously had problems using a PIN.
I don’t know how connections happen with the network.
Is it possible that the same connection identifier is being used between two devices if the SIM is locked on boot?
Perhaps that is why the credentials from one device ended up on another, because the connection was essentially being reused between two different devices.