Nothing you can do about it without modifying engine shaders. Just try to have the transition be as short as possible on that channel to reduce the problem a bit.
This is a png in which there is the blood with the alpha channel. When I drag this decal on to shiny metals materials I continue to get this border. I am pretty sure I am making the alpha map correctly (as this decal looks fine on not-reflective non-shiny surfaces)
The problem seems to be in the metallic property. After lowering the metallic property by .2 the borders were gone. I have no clue why that is. Any Idea?