Why numeric field become TWideStringField after union two datasets in SQLite using TFDQuery

I'm using Delphi 10 Seattle and below is sample codes.

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Data.DB, Datasnap.DBClient,
  FireDAC.Stan.Intf, FireDAC.Stan.Option, FireDAC.Stan.Error, FireDAC.UI.Intf,
  FireDAC.Phys.Intf, FireDAC.Stan.Def, FireDAC.Stan.Pool, FireDAC.Stan.Async,
  FireDAC.Phys, FireDAC.Phys.SQLite, FireDAC.Phys.SQLiteDef,
  FireDAC.Stan.ExprFuncs, FireDAC.Stan.Param, FireDAC.DatS, FireDAC.DApt.Intf,
  FireDAC.DApt, FireDAC.VCLUI.Wait, FireDAC.Comp.UI, FireDAC.Comp.Client,
  FireDAC.Phys.SQLiteVDataSet, FireDAC.Comp.DataSet, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    ClientDataSet1: TClientDataSet;
    ClientDataSet2: TClientDataSet;
    FDConnection1: TFDConnection;
    FDQuery1: TFDQuery;
    FDLocalSQL1: TFDLocalSQL;
    FDPhysSQLiteDriverLink1: TFDPhysSQLiteDriverLink;
    FDGUIxWaitCursor1: TFDGUIxWaitCursor;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  public
    procedure AfterConstruction; override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.AfterConstruction;
var o: TFDLocalSQLDataSet;
begin
  inherited;
  ClientDataSet1.FieldDefs.Add('Code', ftString, 20);
  ClientDataSet1.FieldDefs.Add('Amount', ftFMTBcd, 2);
  ClientDataSet1.FieldDefs.Find('Amount').Precision := 18;
  ClientDataSet1.CreateDataSet;
  ClientDataSet1.AppendRecord(['A', 10]);
  ClientDataSet1.AppendRecord(['B', 20]);
  ClientDataSet1.AppendRecord(['C', 30]);

  ClientDataSet2.FieldDefs.Add('Code', ftString, 20);
  ClientDataSet2.FieldDefs.Add('Amount', ftFMTBcd, 2);
  ClientDataSet2.FieldDefs.Find('Amount').Precision := 18;
  ClientDataSet2.CreateDataSet;
  ClientDataSet2.AppendRecord(['X', 10]);
  ClientDataSet2.AppendRecord(['B', 20]);
  ClientDataSet2.AppendRecord(['Y', 30]);

  o := FDLocalSQL1.DataSets.Add;
  o.DataSet := ClientDataSet1;
  o := FDLocalSQL1.DataSets.Add;
  o.DataSet := ClientDataSet2;
  FDLocalSQL1.Active := True;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if FDQuery1.Active then
    FDQuery1.Close;

  FDQuery1.Open(
    'SELECT * FROM ClientDataSet1 ' +
    'UNION ' +
    'SELECT * FROM ClientDataSet2'
  );
  ShowMessage(FDQuery1.FindField('Amount').ClassName);
end;

end.

Both instance of TClientDataSet have same fields structure (Code is string field, Amount is FMTBcd field).

Why FDQuery1.FindField('Amount') return TWideStringField?