The attached screenshot shows a throttle category set up called "hw1" with a maximum concurrent builds of 8, and a maximum concurrent builds per node of 1

When running with the following Pipeline syntax, everything works just fine:

throttle(['hw1']) {
  node('swarm'){
    sleep 30
    }
}

with 4 nodes labeled 'swarm' this allows 1 job to be executed on each node, and lets us queue an infinite number of jobs that will wait for their turn, and execute with one on each node.

With Declarative pipeline syntax:

pipeline{
  agent {label 'swarm'} stages {
    stage ("have a nap"){
      steps {
        throttle(['hw1']) {
          sleep 30
            }
         }
      }
   }
}

The behavior is that it will fire off 8 jobs, respecting Maximum Concurrent value that 'hw1' was set to, but not the maximum concurrent builds per node, so it fire off 8 jobs, stacking multiple on each node, and only allowing 1 additional job to be queued. Additional triggering of the job does not queue any additional job runs.

Since we really want the entire job limited to 1 execution per node, I was looking around for a way to limit it. https://issues.jenkins-ci.org/browse/JENKINS-45140 describes the issue, and I attempted the solution listed there.

properties([
  [
    $class: 'ThrottleJobProperty',
    categories: ['hw1'],
    limitOneJobWithMatchingParams: false,
    maxConcurrentPerNode: 0,
    maxConcurrentTotal: 0,
    paramsToUseForLimit: '',
    throttleEnabled: true,
    throttleOption: 'category'
  ],
])

pipeline{
  agent {label 'swarm'}stages {
  stage ("have a nap"){
    steps {
     sleep 30
   }
    }
 }
}

Again the same behavior was observed.  8 Jobs fired, multiple jobs stacked per node, and only 1 additional job could be queued