That’s not a bug, thats a feature!
Rhino is not explicitly setting or displaying a tooltip for this field. The OS X framework is deciding to do this. The OS X framework will display this yellow prompt window that contains the full text of a field when the full contents of the field does not fit on the screen. That’s usually a useful feature, but I agree it is annoying here. I’m not aware of a way to turn this feature off, because, although this looks just like a tooltip, it’s actually something else.
Here’s another example. In the Layers panel, reduce the width of the name column until some, but not all of your layer names are truncated. Hover the cursor over a layer name. If the layer name has a trailing ellipsis, the full layer name will show up as a “tooltip”. If the layer name is not truncated, the “tooltip” will not appear. That’s not how “real” tooltips work. Tooltips always appear, whether the field is fully visible or not.