[search API] ASSIGNEE.NOT does not work

under Asana API Explorer

GET /workspaces/XXXXXXXX/tasks/search?opt_fields=gid,assignee,due_on,name,completed,assignee_status&created_by.any=BBBBBB&completed=false&projects.any=AAAAAA&assignee.not=YYYYY,ZZZZZ ASSIGNEE.NOT does not work

Result still contains the parameter passed to the assignee.not

https://developers.asana.com/docs/search-tasks-in-a-workspace
?assigned_by.not string Comma-separated list of user identifiers

Could anyone provide some help?

Any asana professional can help to answer this?