Issue Summary

External system import (CSV) will not add a default value to the custom fields when left empty in the CSV file when updating the existing issues.

Jira version tested : 8.5.3 and 7.13.5

Steps to Reproduce

1. create a custom field (single select)
2. assign a default value to the custom field
3. use the CSV file to import the issues (settings -> system -> external system import) leave the empty value for custom field column in the CSV file
4. map the issue key, summary and custom field columns for importing  

Observe the link https://confluence.atlassian.com/adminjiraserver085/importing-data-from-csv-981155358.html. The section "Updating existing issues" mentions below

(warning) Importing a CSV to update existing issues will reset columns to their default values if they are not specified in the CSV.

Expected Results

The default value of the custom field on the issue ticket has to be updated as per the documentation link

Actual Results

The existing value of the custom field on the issue ticket continues. 

No error is thrown

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available.