With an example where :
Start job triggers in parallel LVL10 and LVL11
Once finished, via a join, Start Job triggers LVL20 and LVL21, and then, in a post join action LVL3
The attached screenshot shows that the whole subtree is shown twice. One for each level1 parallel jobs.
Moreover, if the status is correctly displayed under LVL10, it is not under LVL11.

So, the display has some bugs and is not really what we could await. Something like :

 
Start
|______
    | |----------LVL10
    | |----------LVL11
    =============
    |------------------------LV20
    |------------------------LV21
        |------------------------LV3

would have been easiest to read.
Let's imagine what will happened in complex context with a tenth à parallel jobs at each level, if the complete job cascade at levels n-1 is repeated for each level n paralell jobs !
See the Downstream Build View with a Post Join screenshot.