The latest season of any programme, on first run, usually comes with 30 days of catchup from the date the episode, or the last episode, is shown.
Then they vanish, usually to reappear as part of the box set, once the next season is due to air.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.