I made a new test (https://youtu.be/h5euLYn5VWs), it seems worst with open windows, as we get external noises. I continue not to hear the gas shot, except perhaps when I went from 2nd to 1st twice in the video), but I am absolutely not sure. I would be very happy if you were right and it is because of the sound exhaust that I don't hear (sorry but my last knowledge of AT is old with Jaguar XJ40 in the 90's).
I spoke with the Nissan dealer technical responsible, yesterday and they get an answer from Nissan Europe on Friday at 11h50 am. After analyzing and searching, they don't know this trouble, so they escalated to level 2 which is located to... GB. But he also proposed to install the Consult 3 in my car, make a test and analyze all the downshifts to see if something is wrong.
I think that I'll do that, but as it is 75 km from home, so I have to plan that.
I am satisfied as my dealer followed the problem and seems to treat with serious.
If someone has more ideas or info, I am always open.
Edit 1: I had an appointment yesterday (22 November 2018) with the technician, but at Le Mans, the situation with "Gilets Jaunes" was rather confusing, so I postponed the appointment to next Wednesday (28 Nov).