Possibly nothing wrong with the gearbox.In sport mode you need to give the engine more throttle to make the gear shifts work higher up the rev range.

Edited by elekie&a/c doctor on 31/10/2017 at 19:46

May be an idea to take it to an auto box specialist and ask for an appraisal. If it shifts ok in normal mode,I would be inclined to leave it alone.Any major repairs to the autobox could be more thsn the value of the car.