Your default locale is set to English and the system doesn’t know how to correctly transliterate Vietnamese characters in that case. Either change the default locale site setting to Vietnamese or change the slug generation method site setting to “encoded”.
That’s exactly what’s happening in the app, but you have to do it manually in the rails console.
So, I guess the only thing we should do here is to put a I18n.with_locale(SiteSetting.default_locale) in Slug.ascii_generator, because currently it will use the user’s locale when allow user locale is enabled. Which is kinda weird.