Why cannot take address to a nested local function in 64 bit Delphi?

AS. since closing related questions - more examples added below.

The below simple code (which finds a top-level Ie window and enumerates its children) works Ok with a '32-bit Windows' target platform. There's no problem with earlier versions of Delphi as well:

procedure TForm1.Button1Click(Sender: TObject);

  function EnumChildren(hwnd: HWND; lParam: LPARAM): BOOL; stdcall;
  const
    Server = 'Internet Explorer_Server';
  var
    ClassName: array[0..24] of Char;
  begin
    Assert(IsWindow(hwnd));            // <- Assertion fails with 64-bit
    GetClassName(hwnd, ClassName, Length(ClassName));
    Result := ClassName <> Server;
    if not Result then
      PUINT_PTR(lParam)^ := hwnd;
  end;

var
  Wnd, WndChild: HWND;
begin
  Wnd := FindWindow('IEFrame', nil); // top level IE
  if Wnd <> 0 then begin
    WndChild := 0;
    EnumChildWindows(Wnd, @EnumChildren, UINT_PTR(@WndChild));

    if WndChild <> 0 then
      ..    

end;

I've inserted an Assert to indicate where it fails with a '64-bit Windows' target platform. There's no problem with the code if I un-nest the callback.

I'm not sure if the erroneous values passed with the parameters are just garbage or are due to some mis-placed memory addresses (calling convention?). Is nesting callbacks infact something that I should never do in the first place? Or is this just a defect that I have to live with?

edit:
In response to David's answer, the same code having EnumChildWindows declared with a typed callback. Works fine with 32-bit:

(edit: The below does not really test what David says since I still used the '@' operator. It works fine with the operator, but if I remove it, it indeed does not compile unless I un-nest the callback)

type
  TFNEnumChild = function(hwnd: HWND; lParam: LPARAM): Bool; stdcall;

function TypedEnu