Honestly, I don't think that is a bug. All examples are quite dense and the thumb covers multiple steps on the track. Firefox then translates every click like you clicked directly on the track. The library doesn't interfere with it in any way, it is how Firefox works.
Sometimes it even goes two steps in this implementation. Given that a vanilla <input type="range"> never moves (manually tested in Edge, Firefox, Chrome, Safari now) the handle when clicking anywhere on the handle I would absolutely expect this implementation to work the same.
