I immediately see a thicker blue outline and a second lighter outline of the slice. When you use the slice function it creates new reliefs but does not modify the original relief.
O I selected the reliefs, one by one and arranged them side by side. You can see the flat section we sliced off. The bottom arm is the original and is not modified. Since we don't need it anymore I moved it off to the side.
I then created a second copy of the sliced robot arm relief. I selected this duplicated relief and used the mirror tool to flip it. This reverse relief will be led to the back of the first to create a double sided model of the robot arm.
After dropping all of the reliefs to the bottom of the plate the piece is now ready to tool path and then route.