Flutter Secure Storage not persisting after restarting the app

I'm trying to store some data using FlutterSecureStorage. The data is stored and retrieved correctly every time I open the screen in the app.

However, if I restart the app, the data is lost. What am I doing wrong?

This is how I store the data:

usuario = user.textField.text;
clave = pass.textField.text;
await almacenDatos.write(key: "usuario", value: usuario);
await almacenDatos.write(key: "clave", value: clave);

This is how I retrieve the data.

void credenciales() async {
    String? usuario_aux = await almacenDatos.read(key: "usuario");
    String? clave_aux = await almacenDatos.read(key: "clave");

    if(usuario_aux != null){
      usuario = usuario_aux;
    }

    if(clave_aux != null){
      clave = clave_aux;
    }
  }