Attempting to tween scale a button object scale it continuously.
Tweening the width for example works correctly.
Buttons cannot be scaled. They will automatically adapt their size in a 9-patch fashion and put back their scale to 1. When a scale tween is used, at each frame the button will think that it must grow more and more.
It’s a limitation of the Button custom objects. You can still use width and height tweens as dimension changes are supported.