When the setting forwardshading is selected, Deferred Decal material does not work using basic color.

I created a decal material in UE 4.14, and in project setting, forwardshading is selected. When I link the texture to the material by base color, the decal material does not work at all. Only when I link the texture to the material by Emissive Color, the decal material can work.
When forwardshading is closed, the decal material can work well by link base color between texture and material.