Weird lighting behaviour when mirroring mesh

I searched if there were any similar problems before, but there werent, so I post this question here.

I am not familiar with markdowns. I'll try my best to make this post readable as much as possible.

Here is what happened.

I've constucted a script which flips a given object base on a plane it constucts.

Lots of trial & error there has been, such as flipped faces and so one, but they were well managed. However, one last error left is something I have no previous knowledge about this at all.

Picture upload doesnt seem to work for me so heres a video record: https://youtu.be/lb76iJ2HTa8

As you can see from the video, mirror-copied object receives a ligthing in a weird way.

• Object at the very left is the original.

• Object at the centre is an object copied with Instantiate() method. It receives light, but the shadow is copied as well, which results the shadow lies on an opposite position where it should be.

• Object at the very right is the copied object of the copied object. This is created with new GameObject() , and MeshRenderer and MeshFilter were added afterwards. It has no light at all. I have tested that copying multiple times does not affect the result.

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In case if you think it is because of the baked light, there is only one light in the scene which is directional light, and it is set to realtime. You can see that in the video.

What I would like to happen in my scene is, I hope the object to receive lights properlly. There should be no shadows being copied, or there should be no object which doesnt receive light even if it should be.

If you have ever faced a similar problem like this, but solved it without asking on Unity Answers, please help me out.