sign.update() and setType() probleme

Hello,
My problem is the following :
If I do a setType after a sign.uptade(), the sign update don't work. Why? Is it a bug?

My code :

Code:

public void onBlockRightClick(BlockRightClickEvent event) {
    Block block = event.getBlock();
    BlockState blockState = event.getBlock().getState();
    Player player = event.getPlayer();
    String playerName = player.getName();

    if(block.getType() == Material.WALL_SIGN){
        Sign sign = (Sign) blockState;
        if(sign.getLine(1).equals("")){
            sign.setLine(3, "open");
            sign.update();
        }
        if(sign.getLine(3).equals("open")){
            sign.setLine(3, "close");
            sign.update();

            //Work without this line
            sign.getWorld().getBlockAt(sign.getX(), sign.getY() - 1, sign.getZ()).setType(Material.BEDROCK);
        }
        else if(sign.getLine(3).equals("close")){
            sign.setLine(3, "open");
            sign.update();

            //and without this line
            sign.getWorld().getBlockAt(sign.getX(), sign.getY() - 1, sign.getZ()).setType(Material.AIR);
        }
    }
}

If I switch open to close or close to open and I reboot the serveur, panel display is updated.
An idea?

Sorry if my english is not perfect.