I have a downstream multibranch pipeline a that has a parameter a, and an upstream pipeline b, whose pipeline script is:

pipeline {
  agent any
  stages {
    stage('') {
      steps {
        build job: "a/1.0", parameters: [string(name: 'a', value: 'abc')]
      }
    }
  }
}

I've just created these 2 jobs and branch 1.0 is discovered on initial scan of a, but no builds are triggered. Now I trigger b for the first time, a does not receive the parameter, but if I trigger b again, a receives it, and always receives it in future builds.

As I tested, this does not happen for freestyle project and ordinary pipeline(pipeline that is not multibranch).

Is this a bug?