Rashi on Numbers 12:10/רש"י על במדבר י״ב:י׳

1והענן סר AND THE CLOUD DEPARTED, and only afterwards, ומרים מצרעת כשלג BEHOLD, MIRIAM BECAME LEPROUS, WHITE AS SNOW. A parable! This may be compared to a king who said to his son’s tutor, “Chastise my son, but do not do so until I go away from you, because I feel pity for him” (Sifrei Bamidbar 105).