Hi, the fault can happen hot or cold in any gear for me. I notice it less when the car is going faster because of the speed/momentum. It is the Rev range that is key.
Coventry Audi would not continue investigation on my car because, like you say, no fault codes. They had also done extensive diag on another BiTdi in conjunction with Audi and the conclusion was an engine characteristic. I got a 2nd opinion with the help of Audi UK with Cambridge Audi. Cambridge Audi found that there is an internal bulletin (won’t share a reference) that was released jul2020. It states that they are now investigating a fix as there are now enough of us.
I wanted Audi to extend my warranty for ‘related parts’ but they have refused. Mine is a feb18 car with 49K. It began after a year old, ~20K.
I just purchased a 3yr warranty anyway, as it is becoming a little more frequent.
rang Cambridge Audi last week and there is still no fix recorded. Sorry I can’t give you a reference but this is them keeping it close to their chests