SnowWalkerJ, I have the same problem and it’s the other way round than you guessed:
LSTMCells stacked together by myself as layer for some classification task perform significantly better (6% better in F-Score, tested each with 10 different seeds) than the LSTM module.
I am using LSTMCell because I want to try  a domain adaptation technique by using some procedures at the cell level.
First, I implemented exactly the same model with LSTMCell to see if they perform the same.
Why is the prediction performance gap so huge? Can anyone help?