If a user creates a discount code to reduce the price of the membership to be completely free. Once the member uses the discount code the membership level is set to free however the green notification (alert) box does not appear.
With some testing it would appear that it only disappears with discount codes that make the membership level completely free.
https://www.paidmembershipspro.com/forums/topic/approving-discount-codes-with-no-credit-card-information/#post-92710