Indeed,
I would not draw any conclusions from a single sample.
There could be a bad solder joint, a wronly placed or missing SMT part, bad crysal, ESD damage, a broken PCB track or whatever.
For the average "direct from china" prices you can't be too critical.
Even If 10% would not be working, I'd just consider it acceptable.
You either calculate in some waste, or you pay extra up front for higher quality control on their side.
If however, they advertise as an ST part, and openocd gives you wrong ID's (for multiple PCB's, one can be a fluke), then first complain for a refund, and then still give them full negative feedback for trying to cheat you.