Throughout the Chinese Cultural Revolution, and for some years afterwards, the Square of the Gate of Heavenly Peace was frequently mentioned in the news.
Then at the time of the student democracy protests in 1989, with no explanation that I can remember, we were told that they were concentrated in Tiananmen Square (which most people had great difficulty in pronouncing). It was only some time later that I realized that these two names referred to the same place (I know nothing of Chinese apart from how to anglicize its pronunciation).
The New China News Agency had already decided that from January 1979 all Chinese names would henceforth be reported by them in their Pinyin spellings, and this has gradually filtered through to all English news reports, although in 1989 we were still hearing about Peking rather than Beijing, but this doesn’t seem to me a reason for a translation of a place name to be abandoned in favour of an incomprehensible Chinese name. Could it have had anything to do with the contradiction between its name and the actions of the Chinese government?
For the record, Tiananmen has three syllables, and is most accurately anglicized as /’tj?n śn m?n/ with all four nasals clearly articulated.