Reasonably pleased with my solution. The bitstring packet problems are so well suited to Erlang/Elixir it’s almost not fair.

defmodule Day6 do
  defmodule Input do
    def sample_data(1), do: "mjqjpqmgbljsphdztnvjfqwrcgsmlb"

    def sample_data(2), do: "bvwbjplbgvbhsrlpgdmjqwftvncz"

    def sample_data(3), do: "nppdvjthqldpwncqszvftbrmjlhg"

    def sample_data(4), do: "nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg"

    def sample_data(5), do: "zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw"

    def load() do
      ReqAOC.fetch!({2022, 06, System.fetch_env!("AOC2022Session")})
    end
  end

  defmodule Solve do
    defp find_packet(input, n, i) do
      <<pkthd, pkttl::binary-size(n - 1), rest::binary>> = input

      if <<pkthd, pkttl::binary>> |> uniq_chars?() do
        i
      else
        find_packet(<<pkttl::binary, rest::binary>>, n, i + 1)
      end
    end

    defp uniq_chars?(<<>>), do: true

    defp uniq_chars?(<<a, rest::binary>>) do
      not String.contains?(rest, <<a>>) and uniq_chars?(rest)
    end

    def part1(input), do: find_packet(input, 4, 4)

    def part2(input), do: find_packet(input, 14, 14)
  end
end

Not an issue for the given data sets, but I did just realize this would crash and burn if there was no packet of unique elements of N length. I could handle it but, getting late.