问为什么在填写OnGet时，我的对象在Post上被取消

EN

 [BindProperty(SupportsGet = true)]
        public List<Expenditure> ListExpendituresOver90Days { get; set; }

public async Task OnGetAsync(int projectId, string ExpenditureFilter, string personnelNo)
        {
           ListExpendituresOver90Days = ExpendituresList.ToList();
         }

 public async Task<IActionResult> OnPostButtonAsync()
        {
           return page();
         }

@model WorkInProgress.Pages.Expenditures
<thead>
<tr>
    <th class = text-start>Task Number</th>
    <th class = text-start>Expenditure Type</th>
    <th class = text-end>Item Date</th>
    <th class = text-end>Billable</th>
    <th class = text-end>Vendor Id</th>
    <th class = text-start>Vendor Name</th>
    <th class = text-end>Revenue Amount</th>
    <th class = text-end>Comments</th>
</tr>
</thead>
<tbody>

@for (var x = 0; x < @Model.ListExpendituresOver90Days.Count(); x++)
{
    @Html.HiddenFor(model => model.ListExpendituresOver90Days[x].ExpenditureItemId)
    <tr>
        
        <td class=text-start>@Model.ListExpendituresOver90Days[x].TaskNumber</td>
        <td class=text-start>@Model.ListExpendituresOver90Days[x].ExpenditureType</td>
        <td class=text-end>@Model.ListExpendituresOver90Days[x].ExpenditureItemDate.ToString("dd-MMM-yyyy")</td>
        <td class=text-end>@Model.ListExpendituresOver90Days[x].BillableFlag</td>
        <td class=text-end>@Model.ListExpendituresOver90Days[x].VendorId</td>
        <td class=text-start>@Model.ListExpendituresOver90Days[x].SupplierName</td>
        <td class=text-end>@Model.ListExpendituresOver90Days[x].ProjFuncRevenueAmount</td>
        <td class=text-end>
            <textarea class="form-control" form="MainForm" asp-for="@Model.ListExpendituresOver90Days[x].ExpenditureComment"  rows="1"></textarea>
        </td>
    </tr>
}
</tbody>

<div class="form-control">
    <button type="submit" class="btn btn-primary" asp-page-handler="Button">Save</button>
</div>