Douyin is not the Chinese government. Bytedance is a private company. If their algorithms suck at detecting regional dialects/languages that is not "the evil CCP suppressing minorities". It is just something that Douyin needs to work on and improve. If this is actually true, which i cannot verify.
The reason why i am skeptical if this is that big of an issue is that Douyin is a huge and very widely used platform, and given that there are millions of Cantonese speaking users there would be an absolutely gargantuan outcry against the platform and an issue like this would probably immediately get addressed.