Various examples in documentation and tests either set a hard-coded label for the podTemplate step, which is a race condition waiting to happen, or use ugly Groovy scripting with UUID.randomUUID(). These would better be simplified to use the standard Jenkins environment variable BUILD_TAG, which is unique per build and is also meaningful to humans trying to debug label assignments and node provisioning:

podTemplate(label: BUILD_TAG, containers: [containerTemplate(name: 'ubuntu', image: 'ubuntu', command: 'sleep', args: 'infinity')]) {
    node (BUILD_TAG) {
        container('ubuntu') {
            sh 'whoami'
        }
    }
}

For Declarative, there is no good reason to force the user to define the label at all (see Declarative tests using label: '...'); KubernetesDeclarativeAgent.label could simply be defaulted to something such as BUILD_TAG.