Ash - does not implement `atomic/3`

After doing the Ash Livebook Getting started for several times over the last 2? years,
I finally have a small project that I dare to tackle with Ash.

Loving it so far.

However, sometimes I am not quite sure what to do when receiving an error.

Basic Idea / Context

1. The system is meant to collect “Proposals”.
2. Proposals can be handed in, updated, accepted and rejected during a given timespan.
3. Each given timespan is called a Proposal Round.
4. All Proposals are related to a Proposal Round. They can only be interacted with while the Proposal Round is active.
5. The Proposal Round being active is checked in a custom validation

The custom validation

defmodule Validations.OpenForProposals do
  use Ash.Resource.Validation

  @impl true
  # is this how we would properly check the round_id?
  def validate(%{attributes: %{round_id: id}}, _opts, _context) when not is_nil(id) do
    validate_open(id)
  end

  def validate(%{data: %{round_id: id}}, _opts, _context) when not is_nil(id) do
    validate_open(id)
  end

  def validate(_changeset, _opts, _context) do
    {:error, field: :round_id, message: "No proposal round was set."}
  end

  def validate_open(round_id) do
    with {:ok, round} <- Ash.get(Proposal, round_id),
         true <- round.is_open == true do
      IO.puts("OPEN FOR PROPOSALS") # ---- THIS IS PRINTED :) --------
      :ok
    else
      false -> {:error, field: :round_id, message: "Not currently open for proposals."}
      e -> e
    end
  end
end

Domain Module

defmodule Proposals do
  use Ash.Domain

  resources do
    resource Proposal do
      define :propose,
        args: [:employee_id, :round_id, :text],
        action: :propose

      define :accept,
        args: [:comment],
        action: :accept

      define :reject,
        args: [:comment],
        action: :reject
    end
  end
end

Resource Module

defmodule Proposal do
  use Ash.Resource, domain: Proposals, data_layer: AshPostgres.DataLayer

  actions do
    defaults [:read, :destroy, update: :*]

    create :propose do
      accept [:employee_id, :round_id, :text]

      validate {Validations.OpenForProposals, []}
    end

    update :accept do
      accept [:comment]

      validate {Validations.OpenForProposals, []}

      change set_attribute(:status, :accepted)
      change set_attribute(:comment, arg(:comment), set_when_nil?: false)
    end

    update :reject do
      accept [:comment]

      validate {Validations.OpenForProposals, []}

      change set_attribute(:status, :rejected)
      change set_attribute(:comment, arg(:comment), set_when_nil?: false)
    end
  end

  attributes do
    # ...
  end

  relationships do
    # ...
  end

  postgres do
    # ..
  end
end

The error

The current error is the following:

iex> {:ok, proposal} = Proposals.propose(me.id, 1, "Add 5 more chairs to the break room.")
  {:ok,
   #Proposal<
   ...
  >}
iex > Proposals.accept(proposal, "Good idea. Matthew will take care of it.")
  %Ash.Error.framework{
     errors: [
       %Ash.Error.Framework.MustBeAtomic{
         resource: Proposal,
         action: :accept,
         reason: "Validations.OpenForProposals does not implement `atomic/3`",
         splode: Ash.Error,
         bread_crumbs: [],
         vars: [],
         path: [],
         stacktrace: #Splode.Stacktrace<>,
         class: :framework
       }]}

So the validation works on :propose, but not on :accept.

I have read Basic bulk actions, atomics, new stream options, `error/2` expression and Update Actions — ash v3.29.3, but I do not understand what to do or what the error actually means (and why it occurs on the validation?!)

What am I not seeing / understanding and where I could I learn about it?