DiscountHero removes discount codes only when two things come together: the code carries a restriction, and the customer does not meet it.
When creating a discount code in Shopify, you can restrict it to:
Codes with either restriction are enforced by DiscountHero. The classic case: a new-customer promotion code used by someone who has ordered before.
By default the code is removed. With Smart Discount Replacement, the app can swap it for your existing-customer code instead. See Smart Discount Replacement: swap instead of remove.
Every removal is logged with the code, the removal criteria and the duplicate-check details. See Activity Log and your numbers. If you ever disagree with a removal, that log entry shows exactly why it happened.
Related articles