When performing a search using the != operator, issues with a null value in the specified field is not returned.

For example:
Project is Test
Issue1 has label 'bug'
Issue2 has label 'task'
Issue3 does not have a label set.

Running the query:

PROJECT = 'Test' AND LABEL != 'bug'

This only returns Issue2 and not Issue3

Workaround is to use the query:

PROJECT = 'Test' AND (LABEL != 'bug' OR LABEL is empty)