JQL search showing Incorrect result

Issue Summary

When a user searches for issues not fulfilling a condition from a select field custom field, JIRA shows the wrong results.

How to Reproduce

1. Create a single select list custom field.
2. Add in 3 options (a,b,c)
   • Leave field default as none
3. Add the field to a screen.
4. Create 4 new issues.
5. For each issue, select one of the options for this field
   • Issue 1 = None
   • Issue 2 = a
   • Issue 3 = b
   • Issue 4 = c
6. Go to the issue navigator page Issues > Search Issues
7. Switch to advanced search and add the query below:

   <your customfield> not in (a,b,c)

Expected Result
JIRA would show you issue 1.

Actual Restult
JIRA would show no results.

Workaround

You would need to use an is EMPTY condition to find the issues that have no value set for them.

<your customfield> is EMPTY