Sorry, I’m still not clear on how your master pipeline is working. Are the child pipelines before the Join generating more than one output document? How many documents is the Join outputting? Maybe export the pipeline or take some screenshots and attach it so we can see the details.
You should be able to build the child pipelines so that they don’t have any unlinked outputs. In which case, the PipeExec will only output one document with the status of the child execution. So, as I mention, I’m not clear on why there are multiple documents flowing out of the Join.