If I'm not mistaken, it is by design, the CB has an ability (forgot the name of it) in which the higher the difficulty level the less effective the attacks that is based of its max HP (like poison, HP burn, and anything that is based on it's max HP)

You could check it at the top right corner of the screen everytime you fight it.

Why? To make it harder of course! Because without the ability, players will get "free" damage just because it's HP is significantly higher. Basically you need to land more of such kind of attacks and survive longer.

That is my understanding anyway.

I know that HP burn does the same damage for all difficulties and poison does increasing damage until a point, but then why couldn't they have at least made WM/GS deal the same amount of damage to all difficulties like HP burn or slightly increase their damage per difficulty like poison.