How can pH drop while adding NaOH? Formation and transformation of Mn4(OH)6SO4

It is expected that during a titration using a basic NaOH solution, the pH can only increase. In this article the formation of two different solids during the precipitation of a MnSO4 solution is discussed. The formation of a new compound Mn4(OH)6SO4⋅nH2O instead of Mn(OH)2 is evidenced by the quantification of the remaining ions in solution as well as by X-Ray Diffraction, Raman and infrared spectroscopies. The transformation of this solid into Mn(OH)2 after a sursaturation of the solid in OH- ​can explain a drop of the pH in the titration curve.