Mark Kromer clears up some issues around debugging in Azure Data Factory:
One of the important features built into ADF is the ability to quickly preview your data while designing your data flows and to execute the finished product against a sampling of data prior to finalizing and operationalizing your pipelines.
However, there are a few fundamentals relative to working with Joins that you should keep in mind and a few details below are important to understand at design time and while debugging / testing.
The answer makes sense but it would not have been the first thing to come to mind for me.