If you are cutting this out of plate or flat bar it is difficult to avoid warping. Stainless steel work hardens during the rolling process and there are a lot of stresses in the finished plate or bar. If you only machine on one side the stresses are relaxed unevenly and warping occurs. If you left the radius off you might find the warping is less.
Probably the only fix is to normalize or anneal the material before machining. This does alter the corrosion resistance a bit and may make it more difficult to machine. Annealed stainless is soft and behaves 'gummy' when being machined.