When separating the individual magnets slipped me a out of hand, so that two of them have attracted forcefully. Here, a large piece of the one is chipped, so that formed sharp edges. "Definitely a material error. I still have 9, you can use". But during the day splintered even at two other magnets from the alloy, which the board can get ugly scratches.
Hence my conclusion: Super magnetic force, but simply too strong for the material used. It holds the not stand and shatter too easily.
