Border css variable properties are incorrectly removed

[TychoA]

See the following script:
const less = require('less'); const plugin = require('less-plugin-clean-css'); const P = new plugin({ advanced: true }); const input = '.class {border: 1px solid var(--foo, var(--bar));}'; less.render(input, { plugins: [P] }).then(console.log);
Outputs
{ css: '.class{border:1px solid}', imports: [] }
While it should be
{ css: '.class{border:1px solid var(--foo, var(--bar));}', imports: [] }