Variable alias breaks unrelated detached rulesets call

I have a piece of code that comes down to the following:

@blue: #007bff;
// @primary: @blue;  // Breaks if this line is uncommented

.unlock(@colors, @rules) {
  @rules();
}

.positive(@colors) {
  fg: @colors[dark];
  bg: @colors[light];
}


@fg: { color: @colors[fg]; };


h1 {
  @colors: { light: white; dark: black; };
  @positive: .positive(@colors);
  .unlock(@positive, @fg);
}

Without the @primary definition it works as expected. However when @primary is uncommented I get the following error:

SyntaxError: property "fg" not found in ...:
23   @positive: .positive(@colors);
24   .unlock(@positive, @fg);
25 }

First two lines and the rest of the code are seemingly unrelated to each other, yet these first two lines somehow break the rest of it.

Checked with 4.1.2 (earlier versions have the same issue, at least as early as 3.12.2).

Reproduce the supplied detached-ruleset example with Less 4.1.2, then trace how the @primary alias affects variable lookup and detached ruleset calls. Add a regression test covering both versions of the example and verify that defining @primary no longer causes the unrelated property "fg" error.