Hi ZarbsHow did they test the operation. The MUT tool connected to the OBD port will report any diagnostic error codes for the system. There is a listed diagnostic for when the turn signal will not illuminate or go out but it involves the MUT tool and some tests.
I can sit in my garage with the engine running, apply the turn indicator (left or right) and the signal will be extinguished just by a short turn of the wheel in either direction (at about 11 o'clock and at 1 o'clock respectively) as the wheel is returned to the central position. It doesn't take a lot of movement to disengage the turn signal. I think it is all electric these days like the power steering and there is no mechanism like in the older vehicles where the steering column had a physical bump to disengage the spring loaded lever and return to the central off position.
Looking at the signal mechanism, it simply bolts to the side of the steering column and I surmise the turning of the wheel is picked up by the steering angler sensor mounted on the steering column. I also tested the process today when out and about and the signals were extinguished when the steering wheel was at either 11 or 1 on its return to the central position.
The steering angle sensor needs to be calibrated to find the true centre position but if the calibration were out you would likely see other strange happenings related to the steering and not just issues with the left indicator not going out.
My money is on a faulty turn signal switch which is operating intermittently. I suggest you take the dealer service rep for a ride with you to show him what you encounter with the operation.