The app in the pod is the client (of a DBMS server). The client's IP gets changed. A service in k8s is a network node with an address, but it is used for inbound connections, outbound connections (like from the app to a DBMS server, which may be outside of k8s cluster) usually do not use services (as it gives no benefits).
Usually clients would connect to a Kubernetes svc to not have the problem with changing IPs. Even for just a single pod I would do that.