In Musa's address to his brother Harun, what is the difference between "ابن أم" in Surat Al-A'raf and "يَا ابْنَ أُمَّ" in Surat Taha?

Sheikh Muhammad al-Tahir Ibn Ashour explained the omission of the vocative particle in Harun’s statement to Musa, “O son of my mother, indeed the people oppressed me,” in Surat al-A‘raf, by saying that it was to show the terror and agitation that afflicted Harun, or because this statement came after a preceding statement in which the vocative particle was present, as narrated in Surat Taha, “He said: O son of my mother, do not seize me by my beard.” These are two successive statements, and it appears that what is in Surat al-A‘raf is the second statement, and what is in Surat Taha is what Harun began with.