refresh combobox datasource Bug Declined - Cannot Reproduce

Here I struggle with refresh data source issue:

this is the razor component:

@using Syncfusion.Blazor.DropDowns

@using Syncfusion.Blazor.Popups

@using System.Collections.ObjectModel;

@inject ISecurityService SecurityService

 <div >

     <SfTooltip @ref="TooltipObj" ID="Tooltip" Target=".e-list-item .name[title]">

     </SfTooltip>

     <SfComboBox @ref="comboBoxObj" TItem="EmployeeFullDetailsSearch" TValue="string" AllowFiltering="true" DataSource="@EmployeesTry">

         <ComboBoxEvents TItem="EmployeeFullDetailsSearch" TValue="string" OnClose="OnClose" Opened="OnOpen" Filtering="@OnTypingComboBoxAsync" OnValueSelect="@chooseEmployee"></ComboBoxEvents>

         <ComboBoxFieldSettings Text="FullName" Value="FullName"></ComboBoxFieldSettings>

     </SfComboBox>

 </div>

when typing in the combo box I want to bring new items from my DB (total items 1200K and I don't want to bring all at once).
but it's not working, I don't see the new results in the comboBox I see "NO RECORDS FOUND" but i can see i have items in my EmployeesTry , and it seems the problem is because of async await cause I tried hard code as your example and it worked but bringing new items from DB don't.
I tried both List and ObservableCollection both didn't work.

I also tried to add a ref to the SfComboBox but it didn't work also.

@code {

[Parameter]

Participants participant { get; set; }

[Parameter]

public ObservableCollection Employees { get; set; }

[Parameter]

public GridInjector Injector { get; set; }

[Parameter]

public EventCallback> chooseEmployee { get; set; }

[Parameter]

public EventCallback Filteringhandler { get; set; }

public ObservableCollection EmployeesTry { get; set; }

SfTooltip TooltipObj;

 SfComboBox<string, EmployeeFullDetailsSearch> comboBoxObj;

public Boolean isOpen { get; set; } = false;

private bool intelMode;

protected override async Task OnInitializedAsync()

{

EmployeesTry = new ObservableCollection(await SecurityService.GetEmployeeSearch(""));

}

protected override void OnParametersSet()

{

base.OnParametersSet();

Injector.CompanyChanged = CompanyCheck;

CompanyCheck(participant.CompanyName);

}

private void CompanyCheck(string name)

{

intelMode = name == "Intel";

StateHasChanged();

}

public void OnOpen(PopupEventArgs args)

{

isOpen = true;

}

public void OnClose(PopupEventArgs args)

{

TooltipObj.CloseAsync();

}

protected override async Task OnAfterRenderAsync(bool firstRender)

{

if (isOpen)

{

await TooltipObj.RefreshAsync();

}

}

private async Task OnTypingComboBoxAsync(FilteringEventArgs args)

{

    //await GetEmployees(args.Text);

    EmployeesTry.Clear();

    StateHasChanged();

    var result = await SecurityService.GetEmployeeSearch(args.Text);

    foreach (var item in result)

    {

        EmployeesTry.Add(item);

    }

    await comboBoxObj.RefreshDataAsync();

    StateHasChanged();

}

how can I solve this?