What is the ruling on praying behind someone whose deliberate mispronunciation (laḥn) of the Words of Allah is suspected?

Intentionally distorting the recitation invalidates the prayer. If you are certain that a person intentionally distorts their recitation, then it is not permissible to pray behind them. However, if there is doubt, the default is that the prayer and recitation are valid, and the default is that a Muslim does not intentionally distort the words of Allah. If the matter is merely a doubt or a whisper from Shaytan, do not pay attention to it.