The first thing to check in cases like this is the density of the field point grid.
The first figure below shows a very sparse field point grid. The vertical crosses are the grid points, and the contours are interpolated between the grid points. The second figure below is a grid defined with many more grid points, and with an auto-expansion factor of 1. Since there are more grid points overall, and more grid points in the loaded area, the contouring is much more smooth and appears more circular.
Of course, other loads defined will also affect the loading stress contours.
Sparse field point grid
Denser field point grid