@substring Sorry for the wait. I went ahead and tested it with evtest. Sure enough, when you push and hold down the last button (the eighth button as you described, bottom row on the right) nothing happens. The moment you release the button, it sends both the down event and up event simultaneously. (i.e. value 1 and value 0)
This registers a keypress when you release, obviously, but does not register it when the button is being held down.
All the other buttons are functioning as you would expect in evtest. Value 1 as its being held down, value 0 on release.