I am still experiencing a similar issue with the latest version of the plugin.
On a blank install with only WooCommerce + Advanced Coupons installed and a default theme (Twenty Twenty-Four), when a wrong coupon is added to the cart, the coupon notices are not working as expected.
The issue:
When a wrong coupon code is entered the error message is sent to the Woo notice block at the top of the page and quickly disappears. This unfortunately lets the user without any feedback and screen flickering.
This issue is both happening on the cart page and the checkout page if the Block-Based checkout experience is enabled (which is the default one now)
Expected result:
An error message should be displayed below the coupon field in the wc-block-components-totals-coupon__content > wc-block-components-validation-error DOM element.
Let me know if you need further detail on this issue. Thanks for your help.