Unable to De-select Default Value for Multi Select Custom Field
Symptom
Since JIRA doesn't show "none" on default value configuration page for Multi Select custom field, users may find that as long as they select an option as the default value of Multi Select field, they can't set field default value back to "none".
Resolution
For Multi Select custom field, if you have set an option for it and want to bring it back to "none", you may need to go to default value configuration page, de-select that option by holding "Ctrl" or "Ctrl + Space" and click on the selected option; then save it, you may get default value allocated back to "none" on issue operation page.