Keep decimal values in Reduce function

Your question as it is right now is misleading (I recommend you to change it), the number format is not related to the reduce() function, as Nir said, the graphical representation of your data is not the same as the calculations.

For number formatting with flextable you can use this function

If you need more specific help, please provide a proper REPRoducible EXample (reprex) illustrating your issue.