PBR Graph shader makes flat shaded mesh look smooth.,

So I have a mesh. It's generated during runtime (It is basically a flat plane for now).
Each triangle in the mesh have it's own set of vertices (I decided to make it flat-shaded that way)
I tried to animate this mesh through C# script by changing the position of each vertex over time and then updating it. This way it looks good (flat-shaded low-poly and everything).
This is stupidly inefficient, so I made a vertex displacement shader in shader graph. BUT when I apply this shader to my mesh, it starts to look like it has shared vertices (smooth). Why is that?