In one of my entities, I have a column of type Date and a column of type Fixed Decimal Number. I have explicitly set the data types, and I can see the correct data type icons in column headers.
When I save and refresh my dataflow, then export the JSON file, the data types are set incorrectly to Date/Time (dateTime) and Decimal Number (double), respectively.
As a result, I have to set the data types again in Power BI Desktop.