It does seem to be an issue in our test setup, as it looks like you were suggesting, since if I run the same code outside of the test environment it returns correctly, and the message from IO.puts("after") following the call to the Ecto.Repo.get callback shows up.

I suspect you’re right that the issue is outside of what I’ve shared, but I’m uncertain where to look next.

The test file looks roughly (modified to preserve confidentiality) like:

defmodule Project.ModuleTest do
  use Project.ModelCase

  alias Project.{Module, Factory, TestHelpers}

  @moduletag :module_tag

  setup do
    variables = values

    %{
      more_variables: more values,
      cleanup_func: cleanup_func
    } = TestHelpers.setup_stuff_we_need(variable, other_variable)

    on_exit(cleanup_func)

    built_variables = Factory.build(variables)

    %{
      names: some_variables
    }
  end

  describe "#description" do
    test "test description", %{
      names: some_variables
    } do
      assert {:ok, %{
        specific_thing_we_need_to_check: false,
        id: result_id
      }} = Module.function(thing_id, and, some, values)

      # wait for a different second so records that only have 1 second resolution don't collide
      Process.sleep(2000)

      assert {:ok, %{
        specific_thing_we_need_to_check: true,
        something: ^specific_value,
        thing_id: ^thing_id
      }} = Module.function_that_calls_my_func(thing_id, and, some, values) # <--- the line that actually creates the issue
    end

  end
end

And the file that defines Project.ModelCase looks like

defmodule Project.ModelCase do

  use ExUnit.CaseTemplate

  using do
    quote do
      alias Project.Repo
      import Ecto
      import Ecto.Query, only: [from: 2]
      import Project.ModelCase
    end
  end

  setup tags do
    :ok = Ecto.Adapters.SQL.Sandbox.checkout(Project.Repo)

    unless tags[:async] do
      Ecto.Adapters.SQL.Sandbox.mode(Project.Repo, {:shared, self()})
    end

    :ok
  end

  def errors_on(changeset) do
    Ecto.Changeset.traverse_errors(changeset, fn {message, opts} ->
      Enum.reduce(opts, message, fn {key, value}, acc ->
        String.replace(acc, "%{#{key}}", to_string(value))
      end)
    end)
  end
end

We’ve also got some stuff in config/test.exs, and taking some guesses as to what might be relevant we’ve got

use Mix.Config

config :project, Project.Repo
  adapter: Ecto.Adapters.Postgres,
  username: System.get_env("TEST_USERNAME"),
  password: System.get_env("TEST_PASSWORD") ,
  database: System.get_env("TEST_DATABASE"),
  pool_size: 3,
  pool: Ecto.Adapters.SQL.Sandbox,
  types: Project.PostgresTypes,
  ownership_timeout: 10 * 60 * 1000