On a default Argo CD 3.x install, kubectl shows no health under an Application's status.resources even though the UI shows resource health. Why?

Answer: Resource health is kept outside the Application CR by default

Explanation

Since Argo CD 3.0 the application controller no longer persists each resource's health in the Application CR, because health churn put load on the controller. The status.resourceHealthSource field shows where health lives. Setting controller.resource.health.persist to "true" in argocd-cmd-params-cm restores the old behavior. argocd-server only reads health, it doesn't compute or hold it.