High short circuit failure rate of aluminum electrolytic capacitors during the production processes
That high a failure rate is unheard of for a top-quality supplier like  Nichicon when properly assembled and operated conservatively. Even for no-name parts it’s not at all usual- one in 10,000 might be plausible, but that’s on the high side. Short circuit failures are very rare for aluminum electrolytics. I did once see a few in a bag of 1,000 from a Taiwan supplier that were completely missing the rubber seal so the electrolyte also had gone AWOL- that was actually funny.

You can contact Nichicon directly to confirm the parts are genuine (or not). They may be able to tell just from photos or you might have to courier samples.

You can review their application information to make sure you are not abusing the parts in some way- not only voltage but also ripple current, possible reverse voltage or reverse installation (that is one thing that will cause shorts). Poorly made counterfeits might be marked incorrectly so they are reversed even though they appear to be installed correctly. Your transformerless supply might be stressing the part upon application of power. 

Also confirm that the chemicals and processes used in the PCBA and any subsequent operations such as cleaning are approved. 

I would definitely pull 100% of the parts from that batch of boards and replace them with known good ones. Use good tools and skilled technicians so that reliability isn’t unduly compromised by the rework.  Field failures are extremely expensive in dollars and in reputation. Give them a good visual inspection under a microscope, or at least with a magnifier, and see if you can identify differences between batches or within a batch. 

As far as prevention in the future, that is a bit off-topic, but there are a few approaches to control “quality fade” and substitutions of inferior parts - one of which is third party inspections. The assembly house or supplier you used is suspect if they allowed counterfeit parts to be procured. You may have a better choice of suppliers at higher quantity levels and you can ask how they intend to guarantee genuine parts are used. The Shenzhen markets are a bit of the Wild West, so you need to take care. Nichicon will have authorized distribution channels there, but it’s also possible to procure parts of unknown history at the many retail shops in Huaqiangbei or online at Taobao etc.