SASS Error: The target selector was not found

Question

I'm getting this error:

Error: The target selector was not found.
Use "@extend .animate--fade-in !optional" to avoid this error.
    ╷
276 │     @extend .animate--fade-in;
    │     ^^^^^^^^^^^^^^^^^^^^^^^^^
    ╵

from the following snippet:

.animate--fade-in {
  opacity: 0;
  animation: fade-in 0.1s forwards cubic-bezier(0.19, 1, 0.22, 1);
}

@mixin animate($type: 'fade-in', $time: 1s, $delay: 0s) {
  @if $type == 'fade-in' {
    @extend .animate--fade-in;
    animation-duration: $time;
    animation-delay: $delay;
  }

This worked fine in Ruby Sass, but broke after upgrade to Dart Sass. I cannot understand why it cannot see the selector that is right there with exactly same name. Nothing in the docs hints at what the issue is.

Tried turning .animate--fade-inv into a placeholder #animate--fade-in - no change.