For the metal to flow, would that not mean that it had a space to flow into?
The steel of the die being considerable harder than the bronze of the coin would trap the stamping in the shape of the die.
For the metal to flow and keep its form albeit in a wavy appearance then the pressure of the metal when being stamped would need to be stronger than the hardened steel, pushing the die into a new shape.
is that possible?
Also the waves above the exergue look to me like they have not distorted, which they probably would do if the exergue flowed.
The wavy exergue looks bright, I do not know if this is the lighting when being photographed or a newly cut die being made to fit slightly thicker numbers?