I had my front panel TOUCHPAD STUCK, in the sense it could sense my thumb touch, but NOT my thumb press on the upper-right part (presses on the other directions were detected). I figured out how to fix it:

The touchpad presses are detected by a little sensor on the controller: it’s the little pink button we can see on the picture of Step 8 on the dark-blue board. This button is pressed when pressing the touchpad, via a little black round plastic thingy behind the very center of the touchpad : we can see it on the top of picture of Step 11, the little black circle. If this black circle is off-center or simply *too low*, the contact with the pink button will not happen properly. I personnaly got the black circle thingy off, placed another thin layer of material below it (of approximately the size of the thingy) and placed it back, so it would be more in contact with the pink button. Solved on my both controllers !

Thanks for this really good guide which helped me solve this !