Historically the change of Qiblah from Baitul-Maqdis to Sacred Mosque of Holy Kaabah (Kaaba) occurred in Masjid al-Qiblatain which is also in Madinah. This mosque has two mehrabs for imams. One towards north and the other towards south. Masjid-e-nabavi does not have the two Mehrabs it has just one facing south.
The question is why there is only mehrab in Masjid-e-nabavi. Althouhgt the change of qiblah did not happen there but it is older masjid than Masjid al-Qiblatain. Prophet SA must be praying towards north in the begining. This change must have been done there too.
Is that the Masjid al-Qiblatain’s two mehrabs were kept just to make them memorial?