I don’t reply to these too much, But I remember reading that someone had a similar issue with UE4 ver 4.20.3 but not with 4.18.3, I haven’t read or heard anything on 4.21.2 in concern to the issue. I know with 4.20.3 if I’m making a material and say the values for a color switch are too far apart from one another instead of going from color to color it goes black in between the switch, but if I move them closer together, it doesn’t happen and goes from color to color smoothly with no black. I’m not sure of this kind of display between a range of color(s) would effect what your doing. thought I’d mention it… hope this helped.