Try assembling simple scenes and compile a series of renders.  If you get similar results, fill out a support ticket.  It may be a bug.

Also, DoF is dependent on aperture values (f-Stop) and distance.  The larger the f-Stop value, the sharper the image will be within a distance range.  The lower the f-Stop value, the shallower the sharpness within a distance range.  You will need to consider this when rendering a portrait or landscape.  You will need to increase the pixel sampling to refine the final render, too low and the background will appear too grainy.