Overall I think the reason you’re seeing a jagged edge is due to your lightmap resolution on that particular mesh being too low, but now you’re seeing the difference between static and dynamic shadows. Stationary is a mix of static and dynamic shadows, and if you set that light to movable, you will get a fully dynamic shadow.
To improve the dynamic shadow, I recommend increasing the Dynamic Shadow Distance value, but with everything it’s a balancing act. Dynamic shadows are going to tax the GPU at runtime, while static shadows will be baked to the mesh.