All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no harm in performing an optional prayer in front of al-Mihraab. But, according to the Maaliki scholars, it is disliked for an Imaam to perform an optional prayer inside al-Mihraab.
Ad-Dassooqi said: “Maalik said: 'An Imam should not perform an optional prayer in the place from which he leads an obligatory prayer (i.e. al-Mihraab), but this is permitted for the one who prays alone or is led in prayer.'”
Allah knows best.