Answer
A Historical Work Order doesn't retain a usable Status field to watch for a 'reopened' transition, because reopening a Work Order actually removes its record from the Historical table entirely, rather than just changing a status value on it — trigger the Custom Event on the HistoricalSeparate Logical Unit's 'Object(s) Removed' condition instead, since that removal is exactly what happens at the moment a Work Order gets reopened. If that specific trigger doesn't fit your scenario, a more general fallback is to instead watch the broader WorkOrder Logical Unit directly for changes to its WO_STATUS_ID field.