When tessellation is enabled but the light model is set to "Custom Lighting" the tessellation is not applied when writing to the depth buffer, leading to incorrect shadows and other artifacts when either vertex displacement or Phong tessellation is used. When light model is set to "Standard" the tessellation is applied correctly.
I'm using the newest version v.1.5.4.001, although this has been a problem in the previous versions too. Unity version is 2017.3.1f1.
Below is an image showing shadows bleeding through the tessellated and displaced surface.
The shader is pretty much the simplest possible but I attached it in case you need to take a look.