The direct palette in the chunk data packet no longer writes anything to the packet buffer (instead of just writing a 0); this changed in 17w47a (the flattening). However, they still claim to have a serialized size of 1 byte. This means that the buffer allocated for the chunk data packet has between 1 and 16 extra bytes, one for the palette for each chunk section using the global palette.

In 1.12.2, what MCP calls BlockStatePaletteRegistry has this:

    public void read(PacketBuffer buf)
    {
        buf.readVarInt();
    }

    public void write(PacketBuffer buf)
    {
        buf.writeVarInt(0);
    }

    public int getSerializedSize()
    {
        return PacketBuffer.getVarIntSize(0);
    }

In the current 1.13 snapshots, it instead has this:

    public void read(PacketBuffer buf)
    {
    }

    public void write(PacketBuffer buf)
    {
    }

    public int getSerializedSize()
    {
        return PacketBuffer.getVarIntSize(0);
    }

The return PacketBuffer.getVarIntSize(0) should be changed to return 0.