did you check the readme for 10.15 if there were any changes to this service?
Please check for available Fixes for 10.15 and apply them.
I have found this in the readme for WFF_10.7_Fix9_Readme:
WFF-755 (WFF_10.7_Fix3)
When WmFlatFile config property "recordIdentifierPre105Version" is set to "true"
and Nth field Record Identifier is used,
a) "pub.flatFile:convertToString" adds the Record Identifier to the output.
b) "pub.flatFile:convertToString" does not add the Record Identifier to the
output when the field defined at the same position as Nth field Record
Identifier does not include Record Identifier as value.
The above two issues are now resolved.
Can you check your IS config for FlatFile handling?