I tested the usage of 3 and 4 parts of Plate Armor.
When i used 4 parts, the STAMINA did not increased. Second, i can’t figure by the information given in game, what’s improved from Learning Lv2 > lv3 Plate Armor Set Bonus. Neither other armor types. I have not tested other armor sets.
I tested in a lv50 Priest. No buffs. Plate armor 3 pieces. HP improve = OK. Plate armor 4 pieces. STAMINA INCREASE = NO changes in stamina valor (25 my character stamina).
I have NOT tested the DEFENSE increase of the armor set.

wondering if the mastery bonus is working, unfortunately, doesn’t seems so