The test drive in Finland showed that when three passengers were in the back seats and the car turned sharply, the centre-rear seatbelt failed.
VW said the tests were under “exceptional” conditions and had not been replicated or reported elsewhere.
However, consumer group Which questioned why Volkswagen had sold close to 55,000 additional cars with a “potentially lethal fault” since the problem was first identified in May.
An earlier cable tie solution and an “informal recall” was rejected by regulators.
Alex Neill, Which’s head of home products and services, said: “The decision not to suspend sales when the problem was discovered has now put substantially more drivers, as well as their passengers, at risk.”
Nobody has been injured because of the fault.
The car maker defended its communication of the issue to car dealers and customers as it became aware its fix would not be good enough to satisfy the UK’s Driver and Vehicle Standards Agency.
“These specific communications – both of the issue and the relevant steps to take – with all of our affected customers were felt to be more effective than a statement on a website,” a VW spokesperson said.
“Given the limited circumstances in which the seatbelt can (in the test conditions) come unbuckled, and the employment of the interim fix and further still the specific warnings provided to users, there is no materially increased risk. It is on that basis that sales continued.”
Volkswagen said affected customers would be contacted and could drop their cars off to VW retailers to arrange the fix, free of charge.