Razor Pages model value discrepancy after OnPost modifcation

can somone please help me understand why the page is showing an out of date value for "MyProperty" when using a TagHelper vs a standard input element?

The value is being modified in the OnPost method of a Razor page. After the OnPost the TagHelper input is showing the value that was sent in the post, but the non-tag helper input shows the actual up-to-date value.

Example page model:-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;

namespace WebApplication1.Pages.Test
{
    public class TestModel : PageModel
    {
        [BindProperty]
        public string MyProperty { get; set; } = null;

        public void OnGet()
        {
            MyProperty = "Default value";
        }

        public PageResult OnPost()
        {
            MyProperty = "Posted value overwritten by some logic";

            return Page();
        }

    }
}

Example razor page:-

@page
@model WebApplication1.Pages.Test.TestModel

<form asp-antiforgery="true">

    <p><input value="@Model.MyProperty" readonly /></p>
    <p><input asp-for="MyProperty" /></p>

    <button type="submit">Post data</button>

</form>

On the initial page load (OnGet) the values are the same, but if you hit the submit button to trigger a post, the values rendered in each input are different?