Hi,
Perhaps the resource is remained due to it being deployed as a Custom Resource Definition (CRD).
Mendix for Private Cloud create multiple CRDs to manage mendix app using operator, you can try list it out with this command.
$ kubectl get crd | grep mendix
To delete remaining application pods, you can try:
$ kubectl get mendixapps.privatecloud.mendix.com
$ kubectl delete mendixapps.privatecloud.mendix.com <env-id>
You can also checkout these docs for official refence on how to clean up remaining resources.
https://docs.mendix.com/developerportal/deploy/private-cloud-deploy/#72-deleting-the-cluster
https://docs.mendix.com/developerportal/deploy/private-cloud-deploy/#delete-storage