This is not glitch, nor Corona bug, it's just how smoothing shading works. It's the same in 3ds Max, or in any other 3D program that works with polygonal mesh. There are many ways how to fix this "issue", some of which were already listed above, but the best way would be to learn basic surface shading principles, so you would know what's causing this behaviour and what to do to avoid problems with that in the future.