Follow the ancestral system.
During the Qing Dynasty, princes were also named according to their seniority. For example, Yongzheng’s son was named next to the character for 衣, and Kangxi’s son was named next to the character for 日. This has the same meaning as the seniority in our family tree, it is a convention left by our ancestors. Yongqi's grandson should be named with the character "Xie" next to it, but Mianyi went the opposite way and named his children with the character "金".
Political attack.
Yongqi was deeply trusted by Qianlong, and she once had the intention of letting him inherit the throne. Even if Yongqi died, his child Mianyi would inherit his title, his previous connections and influence. Jiaqing used this pretext to attack Mian Yi, deprive him of his title, deprive him of his power, and drive him out of the political core.
As a punishment.
Mian Yi's behavior actually made Emperor Jiaqing feel threatened. Your father was already deeply loved, but you are so willful and willful that people can't help but speculate. Therefore, in order to show his majesty and to give Mian Yi a warning, Emperor Jiaqing struck him from the head and signaled him to restrain himself.