From several testing I made it seemed that fortify melee potions have no affect in Classic. I used detect mind to check my damage and even with +20%, +50%,+140% total fortify melee by drinking potions, I still deal pretty much the same damage as without them. I tried to create a new save and drunk a small and a strong fortify melee potions and the result was the same, hardly any melee damage bonus. I also tested on another device and it still did pretty much nothing.
I have all the available expansions activated on both testing devices (except Elemental Asteroid), perhaps it had something to do with specific expansions? Since expansions have their unique potions and sometimes can cause a potion from another expansion unavailable to brew, maybe some expansions can mess up the potions from the base game?