The root cause why "Refund" button is not available - payment app should be "Published" only. For unpublished payment app refund is not available, only restock button is available.

Yes. Some time after this post "Refund" button appear - no idea what was changed.

We had similar issue. It's was related to invalid mTLS configuration, but I don't know details. Check this https://shopify.dev/apps/payments/general-transaction-requirements#mtls-configuration, probably it will help

We are a company developing new payment app for Shopify for BNPL payment processing. It's almost ready, and is installed for development store. We are able to pass checkout flow fully (https://shopify.dev/apps/payments/processing-a-payment). It works...