Why does ghunnah appear in incomplete idgham with Waw and Ya, but not with the rest of the letters of complete idgham?

The answer summarizes the reason for the presence or absence of ghunnah (nasalization) in the idgham (merging) of the quiescent noon (نْ) and tanween into the letters of "yarmaloon" (ي ر م ل و ن). Idgham into the letters Lam (ل) and Ra (ر) is not accompanied by ghunnah according to the majority of reciters, though some reciters have affirmed ghunnah in them, the reason being the proximity of the articulation points. As for idgham into the letters of "yanmoo" (ي ن م و), which are Noon (ن), Meem (م), Waw (و), and Ya (ي), it is accompanied by ghunnah. However, some reciters have merged the Noon into the Waw and Ya without ghunnah. The reason for the ghunnah remaining in these cases is the distance between the articulation points, and what the retention of ghunnah signifies about the merged letter. There is agreement on manifesting the quiescent Noon when it meets Waw and Ya in a single word, such as "ṣinwān" (صنوان). The decisive factor in all of this is that recitation is a Sunnah that has been followed from the Messenger of Allah (peace be upon him).