Put_assoc Cannot Perform Update to Many-to-Many

I didn’t think I was doing anything particularly complicated; the ItemA schema looks like

defmodule ItemA do
  use    Ecto.Schema;
  import Ecto.Changeset;

  schema "item_a" do
    …

    many_to_many(:item_bs,                         ItemB,
                 join_through: "item_a_to_item_b", on_replace: :delete);

    timestamps();
  end

  def changeset(%ItemA{} = a, attrs) do
    a |> cast(attrs, […])    # these are actual struct.s from the database, in the code
      |> put_assoc(:item_bs, [%ItemB{}, %ItemB{}, %ItemB{}, %ItemB{}])
  end
end

Yet, after a successful first INSERT, any subsequent UPDATEs DELETEs all entries from the mapping table while the next UPDATE INSERTs them into the mapping table while the next UPDATE DELETEs them while the next UPDATE…

I just want it to get the database to match the modules/structs I give it, not do the literal opposite.