TAIPEI—Benson Chen, a 31-year-old owner of a Taipei-based e-commerce coffee business, recently introduced a newcomer called Meiyu to his family’s online chat group. Trouble followed almost immediately.

Mr. Chen’s mother posted a health advisory offering hydration tips for the flu season. Meiyu weighed in with punctilious certitude.

“This is a rumor,” Meiyu said. “The internet has a lot of information on drinking water. Doctors say not all of it is credible.”

A long silence ensued. “It was awkward,” Mr. Chen said, comparing Meiyu’s rebuke to “an instant slap in the face.”

Meiyu isn’t a pushy new paramour destined for epic mother-in-law battles. She is a fact-checking chatbot, one of the most advanced in the world.

Users have been adding Auntie Meiyu, as the cartoon blonde character is known, to chat groups since she was launched in December. The artificial-intelligence bot will interject in real time when she detects posts about the news, pointing out factual errors and alternative interpretations.

The technology, created by Taiwanese developers, is a step ahead of most fact-checking apps, including versions offered in Brazil and Indonesia, which don’t jump into conversations. Other popular fact-checkers, such as Snopes in the U.S., are public databases that users consult for reviews of news items.

Meiyu quickly became hot in Taiwan, which had just gone through divisive local elections and is rife with rumors of China’s interference in social media. The bot now has more than 110,000 users on the Japanese messaging app Line, which covers about 90% of the mobile-messaging market in Taiwan.

In a nation with long-held Chinese traditions of etiquette, however, Meiyu is proving to be socially inept. Online chat groups often comprise several dozen extended family members. Openly disputing facts with elder relatives is considered bad behavior.

Melissa Edmons said Meiyu has often corrected her mother-in-law in her family chat group. Photo: Melissa Edmons

“False,” Meiyu declares when she detects an errant post. On a post advocating the benefits of eating fruits on an empty stomach, she said: “The doctor quoted in this post does not have proper qualifications.”

Carol Hsu, the app’s developer, said she is trying, after hearing user feedback, to make the bot’s pronouncements sound more conversational. Ms. Hsu also added a boilerplate self-introduction whenever Meiyu is added to a group.

Some are secretly thrilled that Meiyu is speaking up, since they don’t feel they can. Melissa Edmons, a 32-year-old American married to a Taiwanese man, said her mother-in-law used to post up to several times an hour in the family chat group with all manner of dubious tips, from warnings that oven-baked food causes cancer, to instructions on how to survive brake failure in a moving car.

“She’s very forceful about her ideas,” Ms. Edmons said of her mother-in-law. “If we’re trying to correct her, she’ll say, ‘Oh you’re not a doctor, how would you know?’ ”

Ms. Edmons’s brother-in-law introduced Meiyu to the group in late December. Since then, Ms. Edmons estimates the bot has corrected her mother-in-law on half of all her posts, including those on ovens and cars.

“Every time she gets corrected, she kind of doesn’t talk anymore,” said Ms. Edmons, who lives in the southern city of Tainan. “She might stop chatting for a few hours.”

Ms. Edmons said Meiyu would stay in the group. “We’re not removing her. No way.”

Hsieh Fong-chun, a 66-year-old retiree, kicked Meiyu out of his family group after she issued what his nephew, 30-year-old Wellson Wei, said were many corrections to Mr. Hsieh’s posts.

“It’s just weird to have someone outside the family participating” in a family chat, Mr. Hsieh said. “But Taiwan’s a democracy. Everyone’s entitled to their opinion.”

Meiyu, a fact-checking chatbot.

Mr. Hsieh said that when Meiyu was in the group he would listen to the bot if she said something agreeable; otherwise, he would ignore her.

Clyde Lin was scolded by his uncle after the 39-year-old pilot brought Meiyu into the family chat group late last year. “Who is this?” Mr. Lin said his uncle demanded. The family group contained too much personal information to give access to a bot, Mr. Lin said his uncle said.

Mr. Lin was being careless in “inviting a wolf into their home,” he said his uncle told him, questioning the chatbot’s origins.

Mr. Lin said none of his other family members came to his defense. Hours later, his uncle ejected Meiyu from the group—which made things frosty between Mr. Lin and his uncle, he said.

Meiyu created tension in a chat group of 57 gay men that Oliver Lim joined three years ago. When Mr. Lim, a 35-year-old insurance agent in Taipei, added the bot in December, the group’s founder—who wanted his sexuality to remain private to the outside world—viewed her with instant suspicion.

A chat group exchange with Meiyu. After a member posted about the importance of drinking water during the flu season, Meiyu responded: ‘This is a rumor.’ She added links to articles disputing the suggestion, and to an article saying a more effective prevention method is to wash your hands.

“Everyone’s going to get outed this way,” Mr. Lim said the founder told him. “That’s not good.”

Out went Meiyu. Mr. Lim’s relationship with the founder—which had already been deteriorating amid online arguments—hit a breaking point.

“He kicked Meiyu out within probably less than a minute of me adding her,” Mr. Lim said. “He and I are no longer friends.”

Ms. Hsu, the developer, said Meiyu isn’t designed to collect personal data on users. The free app relies on an open-source fact-checking resource in Taiwan called CoFacts.

Meiyu’s perceived rudeness led Chang Shu-huai, who leads a civic watchdog group against fake news, to not recommend her to followers. The group instead stated its preference for three simpler Taiwanese fact-checking apps based on users submitting queries to a database.

Oliver Lim said members of a chat group he belonged to thought including Meiyu was an invasion of privacy. Photo: Oliver Lim

Annie Chang, a 21-year-old student, introduced Meiyu into her chat group in late December because she wanted a nonconfrontational way to correct online falsehoods. It turned out the reaction to Meiyu was much the same as if a live person were speaking, she said.

“It’s like you were on the street one day preaching what you think is a certain truth, and then someone immediately and loudly disagrees with you,” she said.

Meiyu eventually was quietly shoved from her group—Ms. Chang said she doesn’t know who did it—after Meiyu dismissed a post advertising the health benefits of massage.

“The purpose for this chat group was so that everyone can become better friends,” Ms. Chang said. “If I have to keep explaining Meiyu, it’ll just leave a bad impression in the group.”

Write to Chuin-Wei Yap at chuin-wei.yap@wsj.com