Spent a bit of time trying to reproduce this in case it was related to my changes for JENKINS-39203, but was not able to do so. Here is the Pipeline I was using:

pipeline {
    agent any
    stages {
        stage('before') {
            steps {
                echo 'before'
            }
        }
        stage("parallel") {
            parallel {
                stage("simple") {
                    stages {
                        stage ("simple-inner?") {
                            steps {
                                echo 'simple-inner?'
                                sleep 1
                            }
                        }
                    }
                }
                stage("sequential") {
                    stages {
                        stage("sequential-1") {
                           steps {
                                echo 'sequential-1'
                                sleep 1
                            }
                        }
                        stage("sequential-2") {
                           steps {
                                echo 'sequential-2'
                                sleep 1
                            }
                        }
                    }
                }
            }
        }
        stage('after') {
            parallel {
                stage('after-1') {
                    steps {
                        sleep 60
                    }       
                }
                stage('after-2') {
                    steps {
                        echo 'after-2'
                    }       
                }
            }
        }
    }
}

And here is what it looked like:

piratejohnny If you can figure out how to reproduce the problem with tweaks to my basic Pipeline here, that would be helpful. From what I can tell, it looks like what you are reporting might be a dupe of JENKINS-53816, see in particular this comment which notes some problems in this area in Blue Ocean 1.17.0.

If I can find some time Ill create a repro case... 

Here's a clue: I use declarative jenkins pipelines.. however, the "testing" stage calls to a script{} block and inside of there is some groovy scripting that uses the "parallel" command in order to fire a bunch of jobs in parallel.

The defect appears to be that the jenkins declarative parallel build steps dont know how to play nice with groovy scripting that calls for parallel jobs to run.

Interesting that usually when the pipeline is completely finished that things tend to render correctly.. they just dont render correctly while its running causing confusion to the developers who are trying to make sense of whats going on.

the "testing" stage calls to a script{} block and inside of there is some groovy scripting that uses the "parallel" command in order to fire a bunch of jobs in parallel

Ah ok, yeah running stages/parallel blocks inside of script blocks in a Declarative Pipeline generally breaks the visualization, so I'm surprised that it ends up working once the build is complete. Fixing this is probably nontrivial, and might require some substantial changes to the graph generation code in Blue Ocean.

We have a massive jenkins installation and the overwhelming majority of our jenkins pipeline users use declarative scripts and then they use script blocks to call out to subfunctions that often have their own parallel steps. In these cases, the UI rendering is messed up.

From my vantage point, you absolutely need to change that graph algo to be able to render correctly when declarative and scripted parallel steps are mixed.

Hello, I also get the same issue with version : Jenkins 2.235.1 and Blue Ocean 1.23.2. 

Do we have plan to fix the issue? Since it has confused engineers in such case.

-----------------

Update 8/12/2020:

The same issue still exists on our environment.

Jenkins 2.235.1 Blue Ocean 1.24.3

Could you please help to fix it?

Here is what I captured when the job is running:

Here is the status when the whole pipline finished, you can find Job_B marked as failed as expected in the end:

Here is the pipline I use: