Bernadette was definitely incorruptible when exhumed 40 years after her death. Unfortunately, the nuns decided to bathe the body before reburial. When it was re-exhumed a decade later, the moisture was found to have discolored the face so a light coating of flesh colored wax was applied. The wax has no effect on preventing corupption, as embalming fluid would.
If I understand correctly the last time she was exhumed her eyes and nose had begun to deteriorate and her face and hands were turning black. While I understand that wax has no effect at preserving a body - it does cover what is a natural decomposition.