I have set Throttle Concurrent Builds like below:

and my pipeline scripts like below:

//代码占位符
node("m1||travis") {
  stage("Prepage"){
    //some task like git checkout
  }
  stage("Build") {
    //some code like xcodebuild
  }
}

as the code shows, I'm using scripted pipeline. And I found that event though I have set the number of Maximum Concurrent Builds Per Node to 1, Jenkins still build 2 jobs on same node concurrently.