produce_async vs produce_sync partition strategy

I'm trying to use this library in Elixir and while experimenting with the `produce_async` and `produce_sync` functions I found out that

``` elixir
Enum.map(1..10000, fn _ -> :ekaf.produce_sync("partitioning", "test") end)
```

did randomly distribute the messages while

``` elixir
Enum.map(1..10000, fn _ -> :ekaf.produce_async("partitioning", "test") end)
```

does not (all messages seem to end up in the same partition).

Am I missing some configuration? Or is this expected behaviour?