From what I know, their standard for Li is kind of different. They think the Li is identified not by the author but by the reader/player, if a female character describe in detail in the story then the player will start to develop feelings to her(no matter good or bad) and decided she is a Li or not. Moreover, they think if a character is well described in the story then it must be useful in later story(the rule Chekhov's gun), otherwise it will be pointless to make her appear and describe so detail to players.
In Melena's case, a large group of chinese player think she is like a side Li(even she is not in the relationship page) as she is described in the story in detail, related to one of the main Li, have some interested towards mc(even it is only for mc's fame) and may some how affect the story future. Then, after mc send her to leo, in their view is the main character send one of the Li to another male character, which consider as a willingly cuckold aka "送女", become a toxic point for this game and lead to a negative rating.
Btw, if someone still not understand why "送女" is a toxic point for them you need to also understand the definition of main character in their view. The chinese think the main character known as the main character is beacuse the story is mainly focus around the main character, all the character is described as to service the main character just like in the solar system all the planet move around the sun(mc), having another sun in the middle of story is kind of bad(unless the setting is set at the beginning).
Besides, as being the main character they think he is the winner as being the main focus, so either the winner take all or that thing not appear in the story, and the "thing" is including all the female characters. I remeber one of the chinese player common the game in negative rating and describe the atcion of mc towards melena in the word "The mc just like fed the food by others. The food is already in his mouth, instead of chewing and swallowing, he just spit the food into other's dish".
Hope these can make you understand more.