Why does the coupon input field disappear on the checkout page when using third-party discount plugins?

Question

WordPress

When installing any plugin that adds functionality for managing discounts and displays the result of its work on the checkout page, the field with the coupon entry disappears.
Example: the plugin makes a discount for a registered user, subtracting it from the total amount. But if a person also has a discount card, then the discounts must be added up. But the coupon input field is no longer there.
The cart page is not used, only the checkout page. Tell me where to dig?