Weird fade to black on certain color combos

[JacobInCode]

I'm getting an odd low opacity edge on some of my colored border gradients but it doesn't happen with everything

Red-blue works fine but rainbow has the black edge

    'red-blue': ['#f00', '#00f'],
    'rainbow': ['#51E650, #F9D600, #F08E0A , #EC5157 , #FF18BD , #1A4BFF , #62D8F9']