@extended selectors moved with split styles

[dotherightthing]

@Extended selectors are being moved to the split stylesheet when they should be left behind.

In this example, I wish to move only .split-me to split-me.css:

// INPUT

// _demo.scss
%silent_shared {
    color: red;
}
.split-me {
    @extend %silent_shared;
}
.do-not-split-me {
    @extend %silent_shared;
}

// main.scss
@import "demo";

// OUTPUT - expected

// main.css
.do-not-split-me {
    color: red;
}

// split-me.css
.split-me {
    color: red;
}

// OUTPUT - actual

// main.css
// empty

// split-me.css
.split-me, .do-not-split-me {
    color: red;
}