Answer
(Fatwa: 1432/1433=M=1430)
We have written in the previous fatwa that standing in the middle of the row is sunnah for imam. It is meant that the imam is in middle, it is not necessary for him to stand in the mihrab as it is constructed in the middle of mosque; therefore it is said that standing in mihrab is called sunnah, otherwise mihrab means facing mihrab:
کما في رد المحتار: السنۃ أن یقوم الإمام إزاء وسط الصف ألا تری أن المحاریب مانصبت إلا وسط المساجد وھي قد عینت لمقام الإمام الخ
So in the event mentioned in the question, if the imam led the salah opposite to the mihrab in the first row, the salah was valid and the requirement of sunnah was fulfilled though he did so without any proper reason.
But, however it is not known what the muqtadi mean by proper reason, and what reason he gives for condemning imam because of not leading salah from mihrab. If it was explained we would have been able to shed some light on the same as well.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband