I read a paper about Mn doping perovskite, which can improve tolerance to ambient atmosphere by doping Mn2+. But I don't know why Mn2+ is very hard to be directly oxidized.
Mn2+ oxidation may be easier when ozone is used as the oxidant: Mn2+ + O3 + H2O = MnO2 + O2 + 2H+. In the case of high ozone concentrations, the following reaction may even occur: 6Mn2+ + 5O3 + 9H2O = 6MnO-4 + 18H+.
I recommend you to focus on the mixed potential theory to get a deep insight regarding the oxidation process.
Taking a look at the Pourbaix diagram of Mn will help you to get more useful data in this interpretation. As you can see in attached diagrams for Chromium and Manganese Pourbaix diagrams and make a comparison you will understand that stainless steels easily can form a passive layer of Cr2O3 in a wide range of potential and pH, but this range for the formation of MnO2 is very narrow and it is near the water reduction potential line which makes the formation of a stable MnO2 veryt difficult.
Remember that to have an oxide film on the surface of a metal both pH and potentials shall be in the stable range of related Pourbaix diagram and very small changes in theses two factors may change the chemical composition of the oxide layer.
Oxidation process depends on oxidative state of oxidant and other physicochemical properties. After all manganese is heavy metal that become easily reactive in a relatively biological system. It may also by other metals that have similar oxidative state with it.