According to the "Biography of Imperial Concubine Dunsu" in the Qing History Manuscript, because these sons had died before Yongzheng ascended the throne, Fuyi, Fuhui, and Fupei were just their nicknames, and they were not Not yet old enough to enter the family tree and take a name.
Before taking a formal name, there is often a nickname, which is actually very common. For example, Hongli, who later succeeded Emperor Qianlong, was named Yuanshou, and his younger brother Hongzhou was named Tianshen. As for Concubine Nian's three sons, Fuyi, Fuhui and Fupei, they all use the character "福", perhaps because they were born from the same mother.
When they reach a certain age, all the children of the royal family, including the children of some princes, must be ranked according to their age and then recorded in the family tree. Named by the characters.
Some children died young and did not wait for the first teeth to enter the family tree, so they were left with nicknames such as Fuhui, Fuyi and Fupei. Children who grew up successfully had formal names, such as Hongli, Hongshi and so on.
Extended information:
Historically, Concubine Nian was Yongzheng’s favorite concubine. In the 50th year of Kangxi's reign, Nian was married to Yongzheng's side Fujin. She gave birth to the fourth daughter of Yongzheng on March 12, the 54th year of Kangxi. In May of the 59th year of Kangxi, her seventh son Fuyi was born. In October of the 60th year of Kangxi's reign, The eighth son of the emperor was born, Fuhui, and Fupei was born on the tenth day of the fifth lunar month in the first year of Yongzheng's reign.
From her first childbirth to her death, she took over all the children of Emperor Yongzheng, and had no other concubines. From this, it can be seen that Concubine Nian can be said to be Yongzheng's most beloved concubine.
When she was pregnant with the emperor's ninth son Fupei, it happened to be Kangxi's funeral. At this time, there were countless people mourning, kowtowing, and saluting. As she was pregnant, she could not help but feel the fetus. This resulted in dystocia, and Fupei died after birth or was a stillbirth. Her own body also plummeted.
In November of the third year of Yongzheng (1725), Nian became seriously ill and passed away on November 23. Her posthumous title was Princess Dunsu. The emperor stayed out of court for five days to hold the funeral of the imperial concubine. During the funeral, Yongzheng's excitement and unhappy mood can be seen in the "Hanwen Edict" and other documents.
After the death of Concubine Nian, she was posthumously named Imperial Concubine, leaving only the prince Fuhui. Yongzheng loved this son very much, more than other princes. But Fuhui only lived to be eight years old and died. The four children Nian Fei gave birth to all died early, and in the end she was an unlucky person.
Baidu Encyclopedia - Genealogy of Aixinjueluo
Baidu Encyclopedia - Imperial Concubine Dunsu
Baidu Encyclopedia - Aixinjueluo Fuyi
Baidu Encyclopedia-Aixinjueluo·Fuhui
Baidu Encyclopedia-Aixinjueluo·Fupei