From the spec:
"If a <forgiving-selector-list> has an item that contains the nesting selector but is invalid, that item is preserved exactly as-is rather than being discarded. (This does not change the matching behavior of the selector—an invalid selector still fails to match anything—just the serialization of the selector.)"

This is tested in WPT : https://wpt.fyi/results/css/css-nesting/nest-containing-forgiving.html?label=experimental&label=master&aligned