You do not post the value of machineModels from view to hanlder, try to add below code in your form.

<form enctype="multipart/form-data" method="post">
    <div>
        @{ int i = 0;}
        @foreach (var x in Model.machineModels)
        {
            <input type="hidden" name="machineModels[@i].Text" value="@x.Text" />
            <input type="hidden" name="machineModels[@i].Value" value="@x.Value" />
            i++;
        }
    </div>
     Model: <select name="machineModel" asp-for="machineModel" asp- 
       items="Model.machineModels" required></select>
    <input type="submit" asp-page-handler="Download" value="Download 
  certificate" />
</form>

PageModel:

[BindProperty]
public List<SelectListItem> machineModels { get; set; } = new List<SelectListItem>();

I have a simple single page ASP.NET Core Razor app that has a “select” in it that I am populating from a Model variable. The values in the Model variable are cleared by the time that I examine them in the OnPost() method. (The variable named machineModel returns just fine but the variable named machineModels in the Model is cleared by the Post.) What is doing this?

Here’s what the Index.cshtml looks like:

@page
@model IndexModel
@{
   ViewData["Title"] = "Home page";
}
<div class="text-left">
<form enctype="multipart/form-data" method="post">
   Model: <select name="machineModel" asp-for="machineModel" asp-items="Model.machineModels" required></select>
   <input type="submit" asp-page-handler="Download" value="Download certificate" />
</form>
</div>

Here’s what the Index.cshtml.cs looks like:

[BindProperties]
public class IndexModel : PageModel
{
   public string machineModel { get; set; }
   public List<SelectListItem> machineModels = new List<SelectListItem>();

   // In some other function in the class the machineModels variable is filled (several times) ...

   string modelStr = reader.GetAttribute("value");
   int numMachineModels = machineModels.Count;
   string machineModelIndexStr = numMachineModels.ToString();   
   machineModels.Add(new SelectListItem { Text = modelStr, Value = machineModelIndexStr, Selected = true });

   // And here's the Post method ...

   public IActionResult OnPostDownload()
   {
      // Doesn't matter what's in here, machineModels is already cleared at this point

      return Page();
   }
}

Thanks in advance for any help you can provide,
Rich