Hey Herve,
I ran a test on my end and the results I am getting are different than what you are reporting. I think the problem here is that you are adding too many extra variables to the material for testing.
Deferred Decals
Below is a screenshot of my material setup.
Deferred Decal Material
What you could be seeing is either the Screen Fade Size for the decal actor was increased, or your material just has some conflicting inputs which makes your decal appear translucent or render with unexpected results.
Thank you,