CSV import fails to set user picker fields (e.g assignee, reporter, custom field)

Issue Summary

CSV import (through issue navigator) fails to create issue when Reporter, Assignee or User Picker fields are set. Following error appears for validation:

Issue is not created and same error appears if CSV import is performed.

Steps to Reproduce

1. Set a user picker filed/column in CSV file. For example

   summary, reporter, assignee, developer (custom field)
   Test issue, testuser, testuser, testuser
   

   • testuser is username of an existing user, issue persist if email address or accountId is being used instead
2. Perform CSV import through Issue navigator (search issue page) > Import issues from CSV

Expected Results

Issue is created

Actual Results

• Following error appears for validation:
  
• Following error appears if Reporter field is set:

  Issue can't be created due to the error/s [ The reporter specified is not a user.]
  

• Following error appears for User Picker field (customer field):

  Issue can't be created due to the error/s [ User '<accountId>' does not exist. ]
  

Workaround

Import through Jira settings > System > External System Import > CSV Import