Dart: Putting a list into a map shrinks the list

Question

I want to have a map which saves two types of lists. When I try to add a list with two items into a map, the map gets a list with only one item, although it had two! How can I add the list to the map, without losing any items?

    Map<String, dynamic> habitMap = json.decode(oldJsonString);
    List<HabitBuild> habitBuildList = [];
    List<dynamic> dynamicList = habitMap["habitBuildList"];
    dynamicList.forEach((element) {
      habitBuildList.add(HabitBuild.fromJson(element));
    });
    habitBuildList.add(habitBuild);
    habitMap.putIfAbsent("habitBuildList", () => habitBuildList);
    String jsonString = json.encode(habitMap);

Here what says my debugger: