Workpieces are much more likely to twist on a magnet than be lifted off of it. However, there are ways to prevent this from happening.
This wheel mold has only a 1-inch wide annulus for which a magnet can grip, but dowel pins used in select bolt holes will prevent the mold from twisting on the magnet during machining.
The most appropriate workpieces for magnetic chucks are ferrous parts with a lot of surface area for the magnet to grip. Parts lacking a lot of surface area tend to twist on a magnet during cutting operations, especially when cutting forces are directed parallel to the face of the magnet (which occurs when milling about a part's periphery), versus down toward the magnet (as in a drilling operation). In fact, an axial cutting force only 1/18 of a magnet’s total pulling force on a part can cause that part to twist out of position.