Why does my random generation of strings fails?

Let me preface this by saying that I know I can fix my problem by using a library but this is just me doing some elixir exercises for fun. Ok, here is my problem, I want to model a graph and so I created a Vertex struct, this struct has an id property and I want it to be self generating random unique string.

This is my code:

defmodule Vertex do
  defstruct value: nil,
            id: Base.encode16(:crypto.strong_rand_bytes(64)),
            neighbours: MapSet.new([]),

  def new(value), do: %Vertex{value: value}

  def add_neighbour(
      ... Some code here
  end

The unique id generation code is something I copy pasted from a elixir course I took. The problem is that when I try to create vertices I get the same string every time

iex(1)> %Vertex{value: "Bryan"}
%Vertex{
  value: "Bryan",
  id: "59832511CE7A95FFA4BC22FD5A28581DD40AA20C93ACC3E6BAB9B178F6C8D11F3AF14A5EB047846384D5B5CC4B64609C4ABCFFC6D9C4260AA3551A97899253D5",
  neighbours: MapSet.new([]),
}
iex(2)> %Vertex{value: "Cris"}
%Vertex{
  value: "Cris",
  id: "59832511CE7A95FFA4BC22FD5A28581DD40AA20C93ACC3E6BAB9B178F6C8D11F3AF14A5EB047846384D5B5CC4B64609C4ABCFFC6D9C4260AA3551A97899253D5",
  neighbours: MapSet.new([]),
}

But when I execute just the function call it works:

iex(3)> Base.encode16(:crypto.strong_rand_bytes(64))
"16D7500A3A7629E7D83D65E35D3B49CF4D5EB55D3464EFBC44753F268D6C0457C4EC366C7F27CF7BC72A99F8FC7E407BAA9D28E4C39B1F97AB1083174D3C4C1A"
iex(4)> Base.encode16(:crypto.strong_rand_bytes(64))
"D950A48722542628849FD7A0097DE9E7324C4426F6697FEB1A2C35968981FE0B218148CB281BA08B4CEEAC5F3DABAC2C129CF258E466E6FCBF699B2CAA9C7076"

Why is this happening ? It seems like seeding problem but I want someone to explain it to me please.