"During the course of almost two years, 21 employees each took on average 23 days of unpaid leave, the audit said. Their colleagues filled the vacancies by working overtime. "

The issue that needs to be fixed is the unpaid leave.  Management is doing a poor job at controlling its biggest cost, Labor.  Overtime work is inevitable; however excessive unpaid leave that leads to excessive overtime is not.  Management has to stop with the tacit approval and take action.