That Start button trace goes thru 5 vias before getting to the MCU, so I'd make sure the trace run was all good first. Check the Resistance from the top side of C80 to ground, then check it with the button pressed. The value pressed should only be a couple hundred ohms or so. Then check from U6p1 to ground not pressed and pressed, should get the same measurement. If not, could be the trace or one of the 5 vias has an issue. If both measurements are good, then the solder joint at the MCU could be bad or the MCU has an issue.