SHANGHAI — The video app TikTok on Wednesday reversed its decision to block an American teenager who posted a clip in which she discussed the mass internment of minority Muslims in China, and acknowledged that its moderation system had overreached in shutting her out of her account. The incident raised fresh concerns about whether TikTok, which is owned by the Chinese tech giant ByteDance, muzzles its users in line with censorship directives from Beijing — an accusation the company has denied. TikTok said the teenager, Feroza Aziz, 17, had been barred from using her personal device to access the app,...