As you can see the coupon named “ATTESAOLIO” has not been applied. It is a 15% discount restricted to the product with id 19861 that is added to the cart with the url.
If I use the standard cart url, ?apply_coupon works as expected, but we dot not show the cart step in the purchase navigation path.
Just to give you full information:
We have another coupon “SOCIO2020” that is successfully automatically and silently applied using your plugin. In this case the copuon is restricted to the other product in tha cart “Quota associativa 2020” that is automatically added to the cart by this plugin: https://welaunch.io/plugins/woocommerce-product-accessories/