LinkedHashMap copy constructor fails in SwingJS

Test:
    Map<String, String> map = new LinkedHashMap<>();
    map.put("Parent", "ENSG00000123569");
    Map<String, String> copy = new LinkedHashMap<>(map); // or new HashMap<>();
    System.out.println(copy.get("Parent"));
    copy = new LinkedHashMap<>();
    copy.putAll(map);
    System.out.println(copy.get("Parent"));

Prints the expected values in Java, null in JS.