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.
It seems unlikely that you could accidently censor something like this globally without trying to do it for at least one specific target demographic.
It's also plausible that the fault was brought in deliberately by a rogue engineer to raise the subject globally.
There isn't an actual censor who approves stuff, or a list of things that are censored. Companies are expected to reason about it themselves. More broadly, a lot of things are more/less sensitive at particular points in time.
This does suggest that stuff was happening at msft, in anticipation of heightened sensitivity because of the 30 year anniversary.
Doubtful to me, I would think that an action like that would be very traceable in a big corporation like Microsoft
China is known to require search engines operating in its jurisdiction to censor results, but those restrictions are rarely applied elsewhere."