Hey, just ran into the same issue in 4.20 - is there any new solution for that? I found this: https://answers.unrealengine.com/que…on-metals.html - but changing the Metallic property is not a reliable workaround in most cases, unfortunately.
Opacity Mask Clip Value is still not available for deferred decals.
Update: I’ve found out that increasing the Opacity value in the material can fix this to some extent, so that might be a workaround. For example, multiplying the Opacity by 2 or greater value eliminates the white border - but larger values tend to produce artifacts, so still not a perfect solution.