Button bug?

I would like to use button 7L to drive a motor left, and 7R to drive it right. This works, but 8U ALSO drives the same motor???
I have reduced the problem to a few lines. Am I missing something simple, or is this possibly a bug?

#pragma config(Motor,  port3,  TLF, tmotorVex393HighSpeed_MC29, openLoop, encoderPort, None)
#pragma platform(VEX2)
task main()
{ while (true)
  {  motor[TLF] = 127*(vexRT[Btn7L]-vexRT[Btn7R]);
     sleep (25);

I checked that code just to be sure and it works without issue on my test system. Is all the firmware up to date ? (do a software inspection from the robot->advanced menu) Is the ROBOTC firmware that is installed matching the IDE version you are using (latest ROBOTC is V4.55) Are you able to try another joystick, perhaps there is an internal joystick problem?