Resetting a cell with LSTM_cell = LSTM(n_a, return_state = True) is only for debugging purpose. Please remove it when you complete implementation. Here is the thread related to this issue.
In your case, most likely “x” is transformed incorrectly during iterations.
As you see there are multiple steps to transform “x”. It is better for you to check dimensions of “x” on each step, and see which code transforms it incorrectly.
In each iteration,
Input to LSTM_cell : x.shape = (None, 1, 90)
After argmax : x.shape = (None,)
After one_hot : x.shape = (None, 90)
After RepeatVector: x.shape = (None, 1, 90)
You see the shape of the output from the RepeatVector is identical to the input to the LSTM_cell. In you case, most likely those are not.
Hope this helps.