The small trick, which apparently commemorates Martin Luther King Jr Day in the United States, drew criticism from an official Party Weibo account that took issue with Tencent's choice to uphold US holidays rather than Chinese ones.
"US flag icons pour on the chat screen when typing the phrase 'civil rights', but it shows nothing when typing 'National Day', 'China', or 'Five-Starred Red Flag'. Tencent's WeChat team, what are you up to?" the Weibo message said.
This Weibo message has been forwarded over 8,000 times, drawing more than 3,700 comments from users per day after the post appeared on Sunday night.
Each month, WeChat brings new falling emoticons to help celebrate special holidays and festive occasions.
On Monday, WeChat's team apologised and said that the misunderstanding was caused by some backstage setting errors.
In the statement, the WeChat team acknowledged that the new feature aims to mark the birthday of King, but indicated that it was only supposed to be available to US users, and that technical errors resulted in it being available to Chinese users.
"It's not easy for WeChat's internationalisation, and we will worker harder to make our products and operations more considerate in the future," the statement said.
The feature on the messaging app, namely "Falling Emoticons" introduced by WeChat since November 2014, is commonly seen when typing "happy birthday", "miss you", or "XOXO" in all languages, WeChat said in an official English-language blog last year.
Many Chinese people have disagreed with the critique over WeChat's commemoration for King, saying that the official Party account is making a big fuss over nothing.