Just to close the loop here on the public thread:
I had a private conversation with @hriday_m and it turns out the bug is in rnn_cell_forward, but the test cases in the notebook are flawed and do not catch the bug. It is the same bug described on this other thread from today.
I was able to find the bug in the test logic and have filed a github issue with the course staff. I hope they will be able to fix it soon. The test fix is not difficult.