Doodler
You're quite nice when you smile
How does that work? A major feature of accidental lab releases is that they are often not noticed until the broad health implications for the population start to show up in dramatic ways (in this case viral pneumonia and death in quantities that alerted local hospitals). And there is expected to be a big lag between these things, which means by the time the authorities notice, infection is widespread. And this sort of scenario is more likely in a disease with high transmissibility and with a very wide range of outcomes including plenty of mild or asymptomatic cases. All it takes is for someone who works at the lab to get infected and not realise, and they can easily spread it to the community in which they live.
The Russian authorities managed to suppress news of two lab-related anthrax outbreaks, Sverdlosk in 1979 and Kirov in 1953.
On the one hand, the Chinese authorities today are certainly as willing and perhaps almost as able as the old Russian communist party to suppress internal news unfavourable to their interests.
On the other hand, anthrax doesn't spread person-to-person like Covid-19 does, so an anthrax release would be much easier to contain.