Hi Akvan,
When one becomes a Sotapnna, one does not automatically become a Sakadagami Anugami.
– One needs to keep striving. In most cases that is likely to happen.
But there are instances when one attains a magga phala at the dying moment, say the Sotapanna phala.
– But in the next birth that person may be born to a family where Buddha Dhamma may not even be heard about. In that case, one would just be a Sotapanna for the duration of that life.
– Of course, that person will attain the Arahanthood within 7 bhava.