"is there any other table in workflow where we might find work Item ID in relation to the object?"
No, there are no other tables linking objects to workitems. more data might be stored in the worklow container, try using function module sap_wapi_read_container to get the container data or in transaction swia, look at the log->technical display,select the decision workitem (638692), and look at the container tab.
Do you see an invoice type container element? if not the invoice BO is not stored in the decision task and you might have to find the link by the workflow (object->top_wi_id->wi_id of the decision task) or you can change the workflow and add the invoice object to the decision task.