The method Bodies.rectangle calls Vertices.chamfer as found on these lines of code, and it expects that the chamfer property be an object. However, if you pass a single number it will apply the same chamfer number to all of the vertices as seen here.

Reading over the documentation, we found that:

The radius parameter is a single number or an array to specify the radius for each vertex.

For a rectangle, the vertices are clockwise starting from the top left. After all, to chamfer only the top left and top right corners, the code should be:

Bodies.rectangle(x, y, w, h, { 
  chamfer: {
    radius: [5,5,0,0]
  }
})