Thanks, I had exactly the same problem ! Here my new code, if it can help someone esle :)

\[StructLayout(LayoutKind.Sequential)\]
public struct MENUITEMINFO
{
	public uint cbSize;
	public uint fMask;
	public uint fType;
	public uint fState;
	public uint wID;
	public IntPtr hSubMenu;
	public IntPtr hbmpChecked;
	public IntPtr hbmpUnchecked;
	public IntPtr dwItemData;
	public string dwTypeData;
	public uint cch;
	public IntPtr hbmpItem;
}

public class DllImports
{
\[DllImport("user32.dll")\]
    public static extern bool InsertMenuItem(IntPtr hMenu, uint uPosition, uint uFlags, \[In\] ref MENUITEMINFO mii);

    \[DllImport("user32")\]
    public static extern UInt32 SetMenuItemBitmaps(IntPtr hMenu, uint uPosition, uint uFlags, IntPtr hBitmapUnchecked, IntPtr hBitmapChecked);
}

    private void AddMenuItem(IntPtr hMenu, uint id, uint position, string text, Bitmap icon, IntPtr? hSubMenu)
    {
        MENUITEMINFO mii = new MENUITEMINFO();
        mii.cbSize = (uint)Marshal.SizeOf(typeof(MENUITEMINFO)); //48;
        mii.fMask = (uint)MIIM.ID | (uint)MIIM.STRING | (uint)MIIM.SUBMENU;
        mii.wID = id;
        mii.dwTypeData = text;

        if (hSubMenu.HasValue)
        {
            mii.hSubMenu = hSubMenu.Value;
        }

        DllImports.InsertMenuItem(hMenu, position, (uint)MF.BYPOSITION, ref mii);

        if (icon != null)
        {
            DllImports.SetMenuItemBitmaps(hMenu, id, (uint)MF.BYCOMMAND, icon.GetHbitmap(), icon.GetHbitmap());
        }
    }

The biggest problem was this line :

        mii.cbSize = (uint)Marshal.SizeOf(typeof(MENUITEMINFO)); //48;

As in all exemples you found on web, the size is set to 48 by default and not calculate. Thanks again. Decco