When creating a new data Source type - Custom File Export Data Source - and exporting to a specified file the file gets overwritten when over 200 KB.
Product defect. Max File Size value from "Write data to a new file for every export" (default max size being 200 KB) is mistakenly applied to a file defined with "Write data to a specified file".
This issue has been resolved in Workflow/ServiceDesk 8.0 and newer.
At this time, the workarounds involve increasing the size limit.
Alternatively, you can change the value in the console: