When using JQL to search for tickets using the NOT IN() functionality doesn't include tickets that have no value in the custom field or labels field. This is clearly wrong, as when the field has no value, it clearly does not contain the value.

labels not in(productionEmergency)

Expected output: Tickets that don't have the 'productionEmergency' tag

Actual output: Tickets where the labels tag is not empty, and do not have the productionEmergency tag

Workaround: Include the labels clause in a parenthesis, and include OR IS EMPTY

(labels not in(productionEmergency) OR labels is EMPTY)