China’s top streaming platforms censored around six minutes of material from HBO Max’s “Friends: The Reunion” special on the day of its debut Thursday, deleting footage of artists like BTS previously deemed to have “insulted” China, gay fans, references to pee and a shot of Matt LeBlanc in his underwear, among other moments.

“Friends” has amassed a huge, devoted following in China over the years, and thousands of fans had expressed their excitement for the special across all local social media platforms ahead of its release.

But many said they were “bewildered” to find that all three of the country’s major streaming platforms with rights to officially broadcast the show — iQiyi, Alibaba’s Youku and Tencent Video — had entirely cut appearances by musicians BTS, Lady Gaga and Justin Bieber, as well as other scenes.

“Are we reverting to the isolationist Qing dynasty, closed off to the rest of the world?...