So ?...
Where is the problem ?
This was pretty cut n clear since the beginning. And I believe it also has been establish on at least one occasion that this is a fact.
I mean, what did you expect ? Double Barrier ? Come on now, you`re smarter than that.
Its a powerful ability. To such an degree that character with Negate tends to have the other abilities toned down, just due to how good it is.
How did you even arrive at the conclusion that it stacks ?
Almost every thing stack. Why not the barrier? You can have only 5 characters with Negate Damage in the same team and as you said, girls with that tend to be weaker than the others. Five Negate girls would not be that strong if they can't deal heavy damages behind their barriers. And it seems it's not said in the ability description, unlike Sun Altar and Chocolate node ability. It's not stupid to think it can be stacked.
And seriously, most of hardest bosses can easily pass 5 barriers in one turn.

Btw, I'm not looking for a Negate girl espacially. I was looking for Tritonia since she is a Maid Cowslip-like and both of them would be op in the same team with double barrier. But since I can't do double barrier, a Weakness girl seems to be more useful.
For the Rainbow Shards, my current target is Valentine Dendrobium because she is too cute and mofu :3