It works if you provide a generic discovery selector but this should be in the docs:
Example:

spec:
    discovery_selectors:
        default:
        - matchExpressions:
          - key: kubernetes.io/metadata.name
            operator: Exists

Please provide the details on how you installed (helm charts? what was your helm command line? -- using the Kiali Operator? What does your Kiali CR's entire spec.deployment section look like?)

This is already documented here: https://kiali.io/docs/configuration/namespace-management/

I suspect this is not a bug, but your deployment.cluster_wide_access is set to false which is why you need to set discovery selectors.

This does not appear to be a bug. I am moving this to Q&A Discussion. I suspect this is a misconfiguration on the user's part. Please check your "deployment.cluster_wide_access" setting - it is likely set to "false" in which case, yes, you do need to set the discovery_selectors to tell Kiali what namespaces to show. See the documentation referred to in the earlier comment.

Here's replication procedures to see it working - I am using Kiali v2.5.0 as per the OP.

1. Install a cluster and Istio (here I am using the Kiali dev team's hack scripts for this): hack/k8s-minikube.sh start && hack/k8s-minikube.sh istio
2. Install the v2.5.0 Kiali Operator, creating a Kiali CR so it also installs a Kiali Server v2.5.0 at the same time: helm install --set cr.create=true --set cr.namespace=istio-system --set cr.spec.auth.strategy=anonymous --create-namespace --version v2.5.0 kiali-operator kiali/kiali-operator
3. Wait for Kiali Server pod to be ready (you can check via kubectl get pods -n istio-system -l app.kubernetes.io/name=kiali)
4. Check the configuration of the Kiali Server by looking at the ConfigMap directly - look for the deployment.cluster_wide_access and deployment.discovery_selectors - note the defaults are set as expected (which is true and {} respectively):

$ kubectl get configmap kiali -n istio-system -o jsonpath='{.data.config\.yaml}' | yq '.deployment | {"cluster_wide_access": .cluster_wide_access, "discovery_selectors": .discovery_selectors}'

cluster_wide_access: true
discovery_selectors: {}

5. Access the Kiali UI - in this case, a port-forward is required first: kubectl -n istio-system port-forward deployment/kiali 20001:20001 (the UI is then accessible at http://localhost:20001/kiali/console )
6. Confirm you can see the default namespaces in Kiali even if I am a cluster-admin:

The Overview page shows all namespaces

The Namespace drop down menu shows all namespaces: