I have updated plugin. It seems to work if a new account is created as the discount I notice is automatically applied and therefore exhausted for other use. However, since plugin update, I make a new order as guest without using discount, then I make a second order and the discount is still allowed.
Also, the issue with existing customers persists. If a guest customer or logged in customer make a new order, the discount can be used. It seems the code has to be applied at least once to each email address before it is exhausted despite any previous orders