YouTube says it’s begun fixing an error in its moderation system that caused comments containing certain Chinese-language phrases critical of China’s Communist Party (CCP) to be automatically deleted.

The issue meant that comments containing the phrases “共匪” (“communist bandit”) and “五毛” (“50-cent party”) were removed from the site in a matter of seconds. The former phrase is an insult dating back to China’s Nationalist government, while the latter is derogatory slang for internet users paid to defend the CCP from criticism online. It originates from the claim that these users are paid 50 Chinese cents per post.

YouTube told The Verge that the issue that caused comments containing these phrases to be deleted had been fixed for a number of these terms, but that it was still investigating the deeper causes of the error — suggesting other terms may still be affected. In The Verge’s tests, comments containing the two phrases above are no longer deleted from the platform.

The deletions were flagged by users as early as October last year, but YouTube only offered an official response to the problem after it was reported on yesterday by The Verge.

Although the company has blamed the deletions on an error in its “enforcement systems,” it has not explained how this error came to be. A spokesperson told The Verge only that the deletions were not the result of any change to YouTube’s moderation policy, and that the company has been relying on its automated systems more than ever recently as a result of the coronavirus pandemic, implying that the automatic deletions are connected.

However, an increasing reliance on automatic comment filters wouldn’t explain why comments containing these phrases were being deleted six months ago, long before the pandemic began. The deletions also seem particularly strange given YouTube is banned in China, giving the company no incentive to censor anti-CCP comments.

One hypothesis is that the terms in question were added to YouTube’s automated filters as a result of coordinated action by pro-CCP users, who may have mass-reported these terms, triggering some sort of automated response that added them to the platform’s filters.

There have been similar examples of mysterious errors with a pro-CCP bias appearing in Google’s automated systems before. Last year during the Hong Kong protests, Google Translate briefly translated the English phrase “So sad to see Hong Kong become China” into simplified Chinese that read (in English) as “So happy to see Hong Kong become China.” Google quietly fixed the issue without offering any details on the cause.

Without official explanation, politicians in the US including President Donald Trump have leapt on the story and used it to accuse YouTube’s parent company Google of a pro-China bias. For months, now, the White House has pushed a message that US tech companies censor conservative voices and are biased against Republican policies.

Google itself has a long and complicated history with China, with the company showing it’s happy, at times, to censor information in a way that complies with the wishes of the CCP. Most notably, the company developed a censored prototype search engine known as Project Dragonfly for the Chinese market. It was never launched and Google told a Senate Judiciary Committee hearing last year that it had terminated the project.