Issue with Mesh Decals

Hi all,
I have a strange issue with the use of mesh decals, I suspect a bug, since the setup is very straighforward to accomplish this.

I setup my mesh according to the epic doc, nothing complicated about that, juste duplicate some faces, that will receive the decal material and add them to an extra material slot inside my modeler.

In unreal, I usea decal material with *DBuffer Translucent Normal *as the decal blend mode, since I just need the normal channel to engrave a logo in a surface.

My mesh consist in 1 part with a glass material and 1 part with an opaque on > I get a black color, whereas it should not, since I don’t use the color blend mode in the material, just the normal input.
droped the decal actor in the scene to check it, and the look is as intended.

I spend a lot of time today to know where was my mistake, but I still don’t understand why it doens’nt wrok properly.

I also tried to seperate my meshes as 2 differents parts and in this way, eveything is ok : so why in a single mesh there is an issue, and when the meshes are dissociated, it works? Obviously, I need to use my mesh as a single one, so I need to find where is the problem!

Thanks for your help!