ExUnit: Expected behaviour with :group + :async true + :parameterize?

I am trying to run a parameterize’d test with both a :group set and async: true.

The ExUnit docs state: “If both :async and :parameterize are given, the different parameters run concurrently”.

I would expect this to still apply even if a :group is supplied, however, it appears that all parameterized tests use the same group, and they end up running serially.

e.g.

defmodule ParamAsyncGroupTest do
  use ExUnit.Case,
    group: :some_group,
    async: true,
    parameterize: [%{param: :a}, %{param: :b}, %{param: :c}, %{param: :d}]

  test "testing with param", ctx do
    IO.inspect("Testing param: #{ctx.param}")
    Process.sleep(5_000)
    assert true
  end
end

% mix test 
Running ExUnit with seed: 856650, max_cases: 16

"Testing param: d"
."Testing param: c"
."Testing param: b"
."Testing param: a"
.
Finished in 20.0 seconds (20.0s async, 0.00s sync)
4 tests, 0 failures

If I remove the :group then the tests run in parallel:

defmodule ParamAsyncGroupTest do
  use ExUnit.Case,
    # group: :some_group,
    async: true,
    parameterize: [%{param: :a}, %{param: :b}, %{param: :c}, %{param: :d}]

  test "testing with param", ctx do
    IO.inspect("Testing param: #{ctx.param}")
    Process.sleep(5_000)
    assert true
  end
end

%  mix test 
Running ExUnit with seed: 162988, max_cases: 16

"Testing param: d"
"Testing param: c"
"Testing param: a"
"Testing param: b"
....
Finished in 5.0 seconds (5.0s async, 0.00s sync)
4 tests, 0 failures

Thoughts on this?