It does grow back instantly.
You drop below 100

and it just generates out of the void and fills your gaping orifice, satisfying your baser "needs" once again with the conceptual existence that is a diplomacy doubloon.
Do you see that point in space? That one on the other side of the universe? The one that contains nothing at all, and is surrounded by an infinity of other nothings?
That point is exactly equivalent to diplo points. They have accumulated, thanks be to Kestas, in space. There is an infinity of them, praise be to Kestas.