I don't think this is a bug. As you point out yourself, the numberbox has multiple Label's. So you have to target the one you want and hide, change, move etc. that Label. You can hide it with the code snippet above. Also see the adapted code below.
Hi, you can change the View of your Numberbox controller, have a look at the ControlP5customView example. Changing the view allows you to use your own design by keeping the control mechanisms of a controller.