Sass mixin doesn't work in identical conditions

Question

I have found a mixin that worked very well when I initially used it but any subsequent use simply doesn't work, and I'm really failing to see what I am doing wrong. Here's a Codepen I made with snippets of what works and what doesn't.

The c-notification__state is the one that needs to be modified based on state and it doesn't stick, while c-link__link works.

Answer 1

I managed to find a solution, albeit I do not know if it is best-practice, but it now does what it's supposed to. Adding the following right after the @at-root fixed it:

  &--#{$modifier} {
    @content;
  }

I'll leave the Codepen "broken" in case someone would like to mention that I might have done something wrong.