Check that you have the correct orientation of the bone in edit mode.
If its local Y is a different orientation, then you might think you are moving it on the Y but not.
Try moving your driver in all axes and see if this is actually the case.
You can correct the orientation of the bone in the edit mode, and it should work.
If that isn’t what’s happening can you post your file or a screen recording of you showing the process of what you did?
