Basically, the carbon pads in the flex board that makes contact with the buttons increases its resistance with time and makes the button not work properly.
I was able to successfully repair both of them, even though I had some issues with the second one
In this video you can see the process I went through:
This is the link to Aliexpress where I bought the replacement flex boards.
Also, this video was of great help on understanding the issue with this controller's model.