Why is this an issue?

A nested rule made only of & resolves to the same selector as its parent. Wrapping declarations in such a block adds noise without changing specificity, scoping, or behavior.

This rule raises an issue when a nested style rule does not change the selector, such as a bare & { ... } block. The same redundancy applies when the nested block is wrapped in an at-rule.

Noncompliant code example

a {
  & {
    color: red;
  }
}

a {
  @media all {
    & {
      color: red;
    }
  }
}

Compliant solution

a {
  color: red;
}

a {
  @media all {
    color: red;
  }
}

a {
  &.active {
    color: red;
  }
}

Resources

Documentation