PCB cutout specification
One way to handle this is to place a drilled out hole at each corner. Then the milling corner is not a problem. Your cut out corner then ends up looking like the image below. Another alternative is to have the route go past the corner as shown in the second image.



I used a cutout that looked like this for a snap in plastic switch component:



The above style of the cutout was also used on another board design to accommodate a plastic screw standoff that had a rectangular snap-in profile so it would not spin as a security screw was installed.