Of course that makes the assumption that it's known and accepted that Microsoft censor the search "tank man" to some parts of the globe, and the accident is that the censorship applied to a larger than desired scope.
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.
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.
The way chinese censorship works is... erm.. innovative.
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.
Possibly because the filter was updated/refined in preparation for more traffic and new potentially prohibited results, and during that update process the geographic range of the filter was accidentally set to be worldwide.
