The union holds management to a set of policies and there is probably a step-by-step procedure laid out for calling off for sickness with what the employee's re

The union holds management to a set of policies and there is probably a step-by-step procedure laid out for calling off for sickness with what the employee's responsibilities are and the penalty for not following policy, and based on the description the manager probably was holding to policy so the union rep didn't want to fight against the policy. The previous management may have been less strict or forgiving about specific policies (especially with sicknesses). But at the same time if you don't follow the policies evenly, people will start accusing you of favoritism and making up rules as you go along.