@Scott Late to the party but have had the same issue so posting to help others who might have experienced this issue.
Edit the Power Query that is rounding, then click on the Advanced Editor.
In the Advanced Editor looking at the code check for any values that list Int64.Type
In the image below Int64.Type is highlighted as an example.
Int64.Type will automatically round decimal numbers to whole numbers. Changing "Int64.Type" to "type number" (without the quotes) will allow the Power Query to treat the data as a number again.
Hope this helped.