ASP.NET MVC ViewModel not updating in View

I ran into a new and very painful ASP.NET MVC issue.

I was updating a ViewModel property in my Controller, but was not seeing the value updated in my View.

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Manage(ManageAgenda manageAgenda)
{
    // Save an Agenda and get back a sequence-generated Unique ID
    manageAgenda.AgendaId = agenda.AgendaId;
    return View(manageAgenda);
}

On my View:

@using (Html.BeginForm("Manage", "Agenda", FormMethod.Post, new { @id = "ManageAgendaForm"}))
    {
        @Html.AntiForgeryToken()
        @Html.HiddenFor(model => model.AgendaId)
        

Now, this became REALLY painful because my ID kept coming back as 0, so upon each subsequent save, a brand new record was being created ...
The fix for this turned out to be adding the following line of code:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Manage(ManageAgenda manageAgenda)
{
    // Save an Agenda and get back a sequence-generated Unique ID
    manageAgenda.AgendaId = agenda.AgendaId;
    // Send back the Agenda ID.
    // Clear out the ModelState so that the ViewModel will be updated in the View.
    // See: https://stackoverflow.com/a/9645580/463196
    ModelState.Clear();
    return View(manageAgenda);
}

What scares me is I *do not understand* why this works. Other comments just described it as a bug in ASP.NET MVC. As a Software Engineer, I do not like fixes I cannot explain to others or myself.