Nitrogen would work if it were pure. In the real case it may contain traces of air/oxygen, or pick up this imputity as it is being delivered, and Mn(II)  could be sensitive to that. Also the solution may carry air if it has be exposed to said air.

In steel processing applications a mixture of nitrogen with 5% hydrogen is often used, especially for heat treating, with the added hydrogen serving as a "buffer" against air or moisture impurity.