Sorry for my ENG, Im not a native speaker, I mean
1. If one of my brother (0 base melee def) make a shield wall with heater shield, the shield wall buff tooltip write +20 +15 and he is 40 melee def 30 ranged def. When an adjacent brother make shield wall the buff tooltip is +25 +20 and he is still 40 melee def 30 ranged def. I know that’s what you mean.
2.If the same occasion, change the brother to a noble footman, he make a shield wall adjacent to my brother, Im sure the buff tooltip write +20 +15, which means the buff tooltip not gain any bonus……wish you already understand……
I attach a normal adjacent brother bonus to shield wall+25 +20 mentioned above(without shield expert)