Multiclasses / Compound selector missing from output

I'm trying to use the postcss-css-variables to build a CSS file for legacy browsers such as IE using preserved = false.

However I'm getting a inconsistent output while using multi-classes selector .class1.class2 {}. More specifically, the custom property value set inside this type of selector is ignored by the plugin.

Am I targeting something completed off?

Input

:root {
    --BRAND-COLOR--BASE:  #00b7b4;
    --BRAND-COLOR--SHADY: #00382d;
  
    --BRAND-RADIUS--BASE: 3px;
}

.btn {
    --btn-bg-color: var(--BRAND-COLOR--BASE, rgb(200,200,200));
    --btn-color: white;
    --btn-border-radius: var(--BRAND-RADIUS--BASE, 0);
}
.btn:hover {
    --btn-bg-color: var(--BRAND-COLOR--SHADY, rgb(100,100,100));
}
.btn._color_danger {
    --btn-bg-color: #FF6347;
}
.btn._color_danger:hover {
    --btn-bg-color: #FF4500;
}
.btn {
    background-color: var(--btn-bg-color);
    border-radius: var(--btn-border-radius);
    box-shadow: 1px 1px 1px rgba(0,0,0,0.2);
    color: var(--btn-color);
    font-family: sans-serif;
    text-decoration: none;
    padding: 10px 25px;
}

Expected Output

.btn {
    background-color: #00b7b4;
    border-radius: 3px;
    box-shadow: 1px 1px 1px rgba(0,0,0,0.2);
    color: white;
    font-family: sans-serif;
    text-decoration: none;
    padding: 10px 25px;
}
.btn:hover {
    background-color: #00382d;
}
.btn._color_danger {
    background-color: #FF6347;
}

.btn._color_danger:hover {
    background-color: #FF4500;
}

Actual Output

.btn {
    background-color: #00b7b4;
    border-radius: 3px;
    box-shadow: 1px 1px 1px rgba(0,0,0,0.2);
    color: white;
    font-family: sans-serif;
    text-decoration: none;
    padding: 10px 25px;
}
.btn:hover {
    background-color: #00382d;
}

The issue names no source file or test entry point. Reproduce the provided CSS input with postcss-css-variables and inspect how compound selectors such as .btn._color_danger are handled; done means the generated output includes both compound-selector background rules shown in the expected output.