How to handle the value of the inputs with disabled html attribute in Asp.Net core Razore Pages?

When sending an input with disabled html attribute to Update an Entity, I faced with a null input tag when the Modelstate.IsValid was flase.

I want to Update an entity named Episode. My ViewRazore is :

<form method="post" enctype="multipart/form-data">
            <input type="hidden" asp-for="@ViewData["EpisodeId"]" />
            <input type="hidden" asp-for="@ViewData["EpisodeFileName"]" />
            <h2>اطلاعات اپیزود</h2>
            <hr />
            <div class="col-md-8">

                <div class="danger" asp-validation-summary="All"></div>
                <div class="form-group">
                    <label>نام اپیزود</label>
                    <input type="text" asp-for="@Model.Episode.EpisodeTitle" class="form-control">
                    <span asp-validation-for="@Model.Episode.EpisodeTitle"></span>
                </div>

                <div class="form-group">
                    <label>زمان اپیزود</label>
                    <input type="datetime" asp-for="@Model.Episode.EpisodeTimeLength" class="form-control">
                    <span asp-validation-for="@Model.Episode.EpisodeTimeLength"></span>
                </div>

                <div class="form-group">
                    <label>Current EpisodeFileName</label>     
                    <input disabled asp-for="@Model.Episode.EpisodeFileName"  />
                </div>
    </div>
</form>

As you can see, I have the below code:

<div class="form-group">
       <label>Current EpisodeFileName</label>     
       <input disabled asp-for="@Model.Episode.EpisodeFileName"  />
 </div>

I want the Admin to see what the Current EpisodeFileName (it can be an mp4 file or etc) is when updating. Here is my EditEpisode.cshtml.cs:

public class EditEpisodeModel : PageModel
    {
        private ICourseServices _courseServices;
        public EditEpisodeModel(ICourseServices courseServices)
        {
            _courseServices = courseServices;
        }

        [BindProperty]
        public Episode Episode { get; set; }
        public void OnGet(int id)
        {
            Episode = _courseServices.GetEpisodeByEpisodeId(id);
            ViewData["EpisodeId"] = id;
            ViewData["EpisodeFileName"] = Episode.EpisodeFileName;
        }

         
        public IActionResult OnPost(Episode episode)
        {
            if (!ModelState.IsValid)
            {
                ModelState.AddModelError("","لطفا تمام فیلد ها را با دقت وارد کنید");
                return Page();
            }

            return Page();
        }
    }

As you can see, when my application is running, I decided to leave the value of one of my inputs empty, for example I left the following input empty:

<input type="text" asp-for="@Model.Episode.EpisodeTitle" class="form-control">

So as you know, because of my if statment inside the OnPost method:

if (!ModelState.IsValid)
{
     ModelState.AddModelError("","لطفا تمام فیلد ها را با دقت وارد کنید");
     return Page();
}

My OnPost method will call the ReturnPage(). My question is why when the ReturnPage() is called, the input <input disabled asp-for="@Model.Episode.EpisodeFileName" /> become empty!? It's just that, when I use disabled attribute inside my input tag and then post the information, my input is became empty! But when I remove this attribute from the input, I didn't face with this issue ! So anybody can explain it to me ?