When you only cut the cube in half, but leave a force on the full surface of the plate, the unsupported half of the plate tilts and compresses one side of the half cube while putting the opposite side into tension.

To go from a full model with a force of 100 N on the top plate to a half model, you must slice the plates and the cube with the same symmetry plane and you must reduce the force to 50 N since you have only half a cube to compress.