In some cases when the job finishes with status Failed the completedDate is not populated. This happens when the job fails due to connectivity issue for example and the job stays on top of the queue, and the provided Ended Running date is always set to the current date even if the export occurred in the past. Here is the example from the bugfest environment.
GET https://okapi-bugfest-iris.folio.ebsco.com/data-export/job-executions?query=status=(COMPLETED OR COMPLETED_WITH_ERRORS OR FAIL) sortBy completedDate/sort.descending&limit=25
Note that the record listed in the position 0 does not have completedDate.
As a result the only way to delete the job seems to be by removing it directly from the database.