1.Make new project
2.Add object "plane"
3.Add object "cube"
4.Add object "directional light" and turn on hard shadows
5.Check if Player settings option "Use Direct3D 11" is enabled
6.Switch to Android platform or iOS and there will be missing shadows on editor

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By design:

It works the way it supposed to work. When you switch platform to Android or iOS it turns on graphics emulation mode to fit device graphics. Our GLES 2.0 "emulation" clamps the graphics features, which causes the shadows to no longer work (because the desktop versions of the shadow shader code are a good bit more complex than the mobile ones, and they won't fit into the limits imposed by the graphics emulation mode).

If you want to see shadows you can go to Edit>Graphics Emulation and select No Emulation.