Not equals operator (!=) excludes results with empty values in JQL

Expected Behavior

The NOT / != operator in a JQL query should return issues with no value set for the searched field or with an empty value.

Actual Behavior

Issues with non-existing or empty values for the searched field are excluded from the search.

Example #1

• I have the below query on a filter, which returns all issues not assigned to the user admin;

  assignee != admin
  

• This filter returns issues assigned to all users but admin, however it doesn't return unassigned

Example #2

• A JIRA instance has 100 issues;
• 90 issues are under a issue type which should have a field named "Color";
• 80 of those issues have a value for the "Color" field;
• 30 of those 80 issues have the color set to "Red";
• The below query returns 50 issues, whereas it should return 70 issues based on the logical value of !=, which is "not equals to", what should imply "empty" or "null";

  Color != "Red"
  

Example #3

• Below query, should returns all issues with fixVersion not equals to 2.

  fixVersion not in (2) 
  

• but it doesn't return issues with EMPTY values in fixVersion
• Proper JQL query is, which is not intuitive:

  fixVersion not in (2) or fixVersion is EMPTY
  

• Taking into account that IN is alias to (= OR , ...)

Example #4

• Below query, should return all issues with an empty Epic link.

  ("Epic Link" is empty or "Epic Link" != <> 
  

• but it doesn't return all the issues with EMPTY values

Steps to Reproduce

1. Run a filter similar to the below;

   assignee != admin
   

1. Un-assign one of the issues returned by the filter;
2. Re-run the search;

Notice the unassigned issue will be now be missing.

Workaround

Create a filter for issues which have a positive match for the value you are trying to exclude.

Example

1. Create a new filter with the below query and give it a name (e.g. Admin's Issues)

   assignee = admin
   

1. Run a search with the below query;

   filter != "Admin's Issues"
   

Workaround 2

Why my JQL search with “not in” or “!=” (not equals) operators has issues missing

Testing Notes

Using Luke to query the Lucene index directly and searching for -color:red you get 70 issues on the use-case presented above (in "Example #1", under "Actual Behavior").