Big decal causes dark spot on the opposite side of a sphere mesh

[ChiaWeiCharlieWu]

I spawn a decal component on a sphere mesh. When I make the scale of the decal larger, it causes a dark spot on the opposite side of the sphere mesh.

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[ChiaWeiCharlieWu]

I set the radius of Sphere Mask in the material too small. Therefore the scale of the decal is a lot larger than I hoped.

After I adjust the radius of Sphere Mask, It is fine now.