Take a look at the following code:

reminder =
  Ecto.Changeset.change(reminder,
    counter: reminder.counter + 1,
    latest_reminder: DateTime.truncate(now, :second)
  )
  |> Repo.update()
  |> case do
    {:ok, reminder} -> {:ok, Repo.preload(reminder, :user_event)}
    error -> error
  end
  
  # or
  
  reminder =
    Ecto.Changeset.change(reminder,
      counter: reminder.counter + 1,
      latest_reminder: DateTime.truncate(now, :second)
    )
    |> Repo.update!()
    |> Repo.preload(:user_event)

With your code you are passing the tuple {:ok, struct} from Repo.update/2 to Repo.preload/2 instead of just the struct