Variable fallback not working

I'm using postcss-css-variables with a JSON file with this data:

{
    "variables" : {
        "--color-form-selected" : "red",
        "--color-positive-green" : "green"
   }
}

On this css which uses variable fallbacks but oddly it chooses the last variable provided instead of the first.

.a-checkbox__input:checked ~ .a-checkbox__label {
    background-color: var(--color-form-selected);
    border-color: var(--color-positive-green, var(--color-form-selected));
}

But it outputs

.a-checkbox__input:checked ~ .a-checkbox__label {
    background-color: red;
     border-color: red;
}

Expected

.a-checkbox__input:checked ~ .a-checkbox__label {
    background-color: red;
    border-color: green;
}

Use the JSON variables and CSS fallback example in the issue as a regression case, then trace the plugin's variable-fallback handling. Done means the nested fallback preserves the defined green value and produces the expected CSS output without changing the other variable replacement.