Your title refers to Week 2 of DLS C5, but I think you are talking about the Neural Machine Translation assignment in Week 3.
In my notebook, the template code for modelf contains this line before the main loop over the timesteps:
s = s0
Note that it is not capital s. Then at each iteration s is both an input to and an output from the post_activation_LSTM_cell.
That seems completely consistent to me, so am I just missing your point? Or did you perhaps inadvertently modify some of the template code that was given to you? You can always get a clean copy and compare, as described here.