G39 Lambda Sensor Failure

Bought my A7 new in August 2015 and have only ever used Shell V-Power diesel. Despite low mileage (25,400) very few short journeys so "good" miles - it's only used on weekends.

In August 2017, the g39 lambda sensor was replaced (under warranty) after 16,500 miles after emission control system (ECS) light illuminated. Told this was most unusual.

Last week, the ECS light came on again. The car was already booked in for (2nd) service and MOT (at franchised dealership) so this was mentioned when delivering the car yesterday (09/08).

ECS diagnosis revealed same sensor failure, after fewer than 9,000 miles since previous replacement.

Requested explanation for repeated failure of a sensor I have been advised should last significantly longer - at least 50,000 - 60,000 miles but for life of the vehicle if properly maintained & serviced, as mine has. Only got a puzzled look in response. Asked whether or not I should expect to change the sensor every 2 years, irrespective of mileage. Again, no answer came.

Can anyone shed any light on this or advise on possible cause(s)?