This scale armor is obviously missing the pierce damage DT, when you swap it with another armor, it has none.
When you wear it directly, the pierce resistance is not applied also.
Bug solving :
You have to equip it once, then unequip it without releasing it (keep it drag), and put it back in the armor slot, only after this the pierce damage DT applies.
BUT
As soon as you close inventory screen, you loose the piercing resistance.
Obviously bugged