Question about ball lighting's interaction with AoE damage

Title explains most, but after putting many hours into a ball lightning build I just noticed that the tooltip doesn't change when increasing area damage.

Is this just one of those things that is invisible to the tooltip, or is this some kind of bug?

It is not a bug, ball lightning does not deal area damage. It is tagged with aoe purely because the range at which it strikes enemies is affect by modifiers to area of effect.

I can see a reasonable argument against adding AOE damage to ball lightning.

Ball lightning gets more damage as AOE increases because it starts hitting mobs earlier and stops hitting them later. This means that AOE expansion + AOE damage would be pretty hard to balance for ball lightning because of how AOE itself adds quite a bit to ball lightning damage.

The argument is pretty simple actually, its based on definition of area damage

area damage covers all the area with damage, but ball lighting does not do that, it hits individual targets within the radius - therefor, its not area damage