A lot of terms in China only get censored around the sensitive dates. Or, to put it differently, around the time that there is a sensitive date (PRC anniversary, CCP anniversary, June 4 etc) there seems to a burst of newly-censored terms. Because there is no official list of banned terms, this is done proactively by censors in the tech companies who want to avoid a potential warning from the government or pile-on from nationalist netizens.
It's possible that the English term "tank man" wasn't censored on Bing image search in China before, but it is now. Over there the Tiananmen massacre is usually referred to as the June 4 incident, so it's usually the characters 六四 (6 and 4) that are censored in search results. Because Bing isn't a very popular website in China, it might be that "tank man" slipped through until now.