When you can't prove it works, just make it not working !
As Buttons are inputs, I did not set levels on the corresponding pin of the I2C controller, to avoid forcing levels on the touch button output.
So, to make it not working, I tried to force the signal during setup to HIGH level, then everything is working fine. I can read the diagram of the I2C controller but I can't find why there is this kind of "Strong" low level priority.