C# ASP.NET Core 9 MVC: invalid ModelState on list properties when posting my ActionResult

I am trying to create a registration form (view). Whenever I do a HTTP POST with my model, I get an "Invalid ModelState" error on the 2 lists that I have in my view model. All properties are filled - the error is on the second list - not the data keyed!

I am using ASP.NET Core 9.0.2.

Here is my view model:

public class RegisterViewModel
{
    [Required]
    [EmailAddress]
    [Remote(action: "IsEmailInUse", controller: "Account")] // Remote validation
    public string Email { get; set; }

    [Required]
    [DataType(DataType.Password)]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password",
       ErrorMessage = "Password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }

    [Required]
    [Display(Name = "First Name")]
    public string FirstName { get; set; }
    [Required]
    [Display(Name = "Last Name")]
    public string LastName { get; set; }

    [Required]
    public bool Terms { get; set; }

    public List<SelectListItem> User_Types { get; internal set; }
    [Display(Name = "Membership")]
    public string User_Type { get; set; }

    [Display(Name = "How Did You Hear About Us?")]
    public String User_HDYHAU { get; set; }

    public List<SelectListItem> List_HowDidYouHearAboutUs { get; internal set; }
}

Here is my controller - on the Post, I am only showing the first few lines as the ModelState shows invalid right away. Notice that I have tried to repopulate the 2 lists already:

[HttpGet]
[AllowAnonymous]
public IActionResult RegisterCreate()
{
    var List_HDYHAU = GlobalHandlers.GetListOf_HDYHAU().ToList();

    RegisterViewModel model = new RegisterViewModel()
    {
        User_Types = (from c in GlobalHandlers.GetListOf_Roles().ToList()
                      select new Microsoft.AspNetCore.Mvc.Rendering.SelectListItem
                      { Value = c.Key, Text = c.Value }).ToList(),

        List_HowDidYouHearAboutUs = (from c in List_HDYHAU
                                     select new Microsoft.AspNetCore.Mvc.Rendering.SelectListItem
                                     { Value = c.Key, Text = c.Value }).ToList(),
    };

    return View(model);
}

[HttpPost]
[AllowAnonymous]
public async Task<IActionResult> RegisterCreate(RegisterViewModel model)
{
    var List_HDYHAU = GlobalHandlers.GetListOf_HDYHAU().ToList();
    model.List_HowDidYouHearAboutUs = (from c in List_HDYHAU
                                       select new Microsoft.AspNetCore.Mvc.Rendering.SelectListItem
                                       { Value = c.Key, Text = c.Value }).ToList();
    model.User_Types = (from c in GlobalHandlers.GetListOf_Roles().ToList() select new Microsoft.AspNetCore.Mvc.Rendering.SelectListItem { Value = c.Key, Text = c.Value }).ToList();

    bool errorExists = false;

    if (ModelState.IsValid)
    {
        // .....MORE CODE HERE
    }
}

What am I missing ? Thank you very much.

I tried to repopulate the 2 lists on the post and update the model, but that is not working.