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A data engineer at an HR analytics company is developing a PySpark pipeline to analyze salary metrics across departments. They wrote the following line of code to compute the total, average, and count of salaries per department: result_df = df.groupBy("department").agg({"salary": "sum", "salary": "avg", "salary": "count"}) After running the code, they observed that the resulting DataFrame only contains one aggregated value instead of the three expected metrics. What is the most probable cause of this issue?