Download column is empty for a flex datasource.
Audit report lists download column as a negative number.
Flex config can hard set the total bytes using value val_bytes=1 because bytes is a required field and in this instance was not available to the datasource.
Change the flex configuration setting the variable val_bytes=0 instead of val_bytes=1.
Test as a new datasource first. Any existing datasources can be modified by engineering once testing confirms the resolution.
The download field is calculated by subtracting the upload data, from total data (bytes) which will be a negative number if the value is 1.