Unable to import custom fields from csv for issues

[Deleted user]

I'm importing a large CSV file with issues from Gitlab. 

In Jira i've created the following fields under issue type "task" and this type is set as default: 

- Gitlab Issue ID

- Gitlab Author

- Gitlab Assignee

Mapping this fields during the import isn't a problem and everything gets imported. 

When i check the imported issues, the custom fields are all empty. 

CSV style: 

Issue ID,URL,Summary,Status,Description,Author,Author Username,Assignee,Assignee Username,Confidential,Due Date,Created At,Updated At,Closed At,Milestone,Labels

409,https://code.gitlabserver.com/projectname/app/issues/409,Update of the Editor image library,"Open","Users were complaining the current image library wasn't functioning properly. I designed a proper working image library. You can find the prototype here: https://url.here.com/
The sketch file can be found here: https://url.here.com/",Dave Hoeks,daveh,,,No,,2017-02-28 12:30:02,2019-01-16 10:05:04,,,Prio-3 editor features

Expected: In this example only Gitlab Assignee has to be empty. 

Reality: Gitlab Author and Gitlab Issue ID are empty too. 

Created at, Updated at, Labels, Status etc. are filled in correctly after the import.  

[Pete Singleton]

I can't see in the CSV any values for 

- Gitlab Issue ID

- Gitlab Author

- Gitlab Assignee

Where are you mapping these from in your CSV file?

[Deleted user]

At Import > Setup > Fields: 

Issue ID -> Gitlab Issue ID;

Author -> Gitlab Author;

Assignee -> Gitlab Assignee;

At Import > Setup > Fields > Values: 

Assignee
(Imported as Custom Field Gitlab Assignee)

The values displayed at this screen are correct.