Quoting from https://jenkins.io/doc/book/pipeline/syntax/

agent { node { label 'labelName' } } behaves the same as agent { label 'labelName' }, but node allows for additional options (such as customWorkspace).

If I use the longer syntax, then the Blue Ocean Pipeline Editor shows the label in "Pipeline Settings". However, if I use the shorter syntax, then the "Agent" drop-down list just shows "Select an option". The UI should be the same in either case.

Test with Jenkinsfile:

pipeline {
  agent {
    label 'labelName'
  }
  stages {
    stage('Build') {
      steps {
        sleep 1
      }
    }
  }
}

vs.

pipeline {
  agent {
    node {
      label 'labelName'
    }
  }
  stages {
    stage('Build') {
      steps {
        sleep 1
      }
    }
  }
}

IMHO, it would be OK to silently convert one syntax to the other.