I think I’ve noticed a pattern when it comes to on’yomi with Chinese counterparts ending in
“-ng”. If the Chinese word ends in “-ing” then the corresponding on’yomi will have a similar initial consonant and be followed by えい or ょう. (Ex. 名 ming -> めい/みょう or 生 sheng ->せい/しょう and the list goes on) If it ends in “ang” or “ong” it will end in おう (Ex. 常 chang -> じょう or 同 tong -> どう). I’m sure there is a pattern with “eng” endings as well, but I don’t know it. Is there a linguistic reason as to why this is seemingly the case?
My theory has always been that it disappeared as the Japanese language evolved.