It seams that something changed in 4.13 with rendering decals.
In 4.12 I can use Actor Location with decals to make gradient around the center of decal with this nodes:
In 4.13 and 4.14 preview Actor positions always returns 0, also given mode (Blend mode=opaque, Decal Blend mode = translucent) is no longer possible to set.
It generates problems with volumetric deals in which Actor position is used to calculate ranges or mapping.