Why does the DropDownListFor lose the multiple selection after Submit but the ListBoxFor doesn't?

Question

I have read many articles about using MultiSelectList and have yet to understand what is going wrong with my DropDownListFor. I have a ListBoxFor with the same View, ViewModel and data that works fine. I want to use the DropDownListFor because of its optionLabel parameter that ListBoxFor doesn't have.

When the View is first loaded, both the DropDownListFor and the ListBoxFor show the multiple selected items.

When the Submit button is clicked, the selected items collection is posted back to the Controller action okay and the view is refreshed with the ListBoxFor still showing both selected items but the DropDownListFor is only showing one selected item.

The controller action is constructing the MultiSelectList like this:

vm.TasksFilterGroup.Assignees = new MultiSelectList(employees, "Id", "FullName", new string[] { "51b6f06a-e04d-4f98-88ef-cd0cfa8a2757", "51b6f06a-e04d-4f98-88ef-cd0cfa8a2769" });

The View code looks like this:

<div class="form-group">
  <label>ListBoxFor</label>
  @Html.ListBoxFor(m => m.TasksFilterGroup.SelectedAssignees, Model.TasksFilterGroup.Assignees, new { @class = "form-control", multiple = "multiple" })
</div>
<div class="form-group">
  <label>DropDownListFor</label>
  @Html.DropDownListFor(m => m.TasksFilterGroup.SelectedAssignees, Model.TasksFilterGroup.Assignees, new { @class = "form-control", multiple = "multiple" })
</div>

Why does the DropDownListFor lose the multiple selection after Submit but the ListBoxFor doesn't?