A process that normally lowers the corrosion resistance had the opposite effect when applied over an alternate known-good option. Chromium passivation is the known-good, manganese passivation is known to make it worse. Manganese-passivating chromium-passivated stainless looks like it's way more corrosion resistant than either passivation on its own and non-passivated stainless
Also, daily reminder, stainless steel corrodes. It forms a corrosion layer that is hard and doesn't change size compared to the base metal (unlike rust on normal steel, which expands), but it can still be prone to erosion particularly in oxygen-depleted water. Even 316.