C++ : Why this window title gets truncated?

Visual C++ 2012 RC, Win7

Chinese simplified

Project Properties > use multi byte character set

When I run this program, the window's title shows a single letter "S", not a whole word "Sample".

#pragma comment(linker, "/SubSystem:Windows")

#include <windows.h>

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE, PSTR, int) {
    WNDCLASSW wc = { 0 };

    wc.style            = CS_VREDRAW | CS_HREDRAW;
    wc.hInstance        = hInstance;
    wc.hIcon            = LoadIcon(nullptr, IDI_APPLICATION);
    wc.hCursor          = LoadCursor(nullptr, IDC_ARROW);
    wc.hbrBackground    = reinterpret_cast<HBRUSH>(GetStockObject(WHITE_BRUSH));
    wc.lpszClassName    = L"MyWindowClass";

    wc.lpfnWndProc = [](HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
        if (uMsg - WM_DESTROY)
            return DefWindowProc(hWnd, uMsg, wParam, lParam);
        else {
            PostQuitMessage(0);
            return HRESULT();
        }
    };

    RegisterClassW(&wc);

    CreateWindowExW(0, L"MyWindowClass", L"Sample",
        WS_OVERLAPPEDWINDOW | WS_VISIBLE, CW_USEDEFAULT, SW_SHOW, CW_USEDEFAULT, 0,
        nullptr, nullptr, hInstance, nullptr);

    for (MSG msg; GetMessage(&msg, nullptr, 0, 0); DispatchMessage(&msg));
}

If I use Unicode (Project Properties), keep source code unchanged, window title shows "Sample", looks correct.

If I use multi byte, in source code I use WNDCLASS = { ..., "MyWindowClass" } and RegisterClassA, keep CreateWindowExW unchanged, window title shows word "Sample", looks correct.

If I use multi byte, in source code I use CreateWindowExA("MyWindowClass", "Sample"), keep WNDCLASSW and RegisterClassW unchanged, window title shows letter "S".

What makes it show a single "S", am I doing something wrong?

Append

If I keep all unchanged, that is, use multi byte, use code shown above, window title shows letter "S".

(If you run this program and see "Sample" on window title, rather than "S", then it's more likely a specific problem on chs version of vc++ 2012 (or OS)).