A refund can be stuck in pending for 24-48 hours depending on how long it takes to process the refund. ie. some credit card companies take longer than others.
If it's longer than 48 hours, the refund can get stuck on Amazon’s side if the refund is triggered but they have not fulfilled the order yet, which means the customer has not been charged yet.
You can go ahead and cancel the order in Amazon to resolve this.
For further assistance, please reach out to marketplace-connect@shopify.com