The reason municipal flags in China are so rare is that the central government officially banned them in 1997. Nanjing is one of the cities that complied, abandoning its eleven-year old flag. The seal on the flag depicted a bixie standing over the city’s Zhongshan Gate.
The ban has either been ignored or forgotten in recent years as Chinese cities have started creating new flags. Kaifeng adopted its flag in 2006, and Shangrao followed suit in 2009.