AFAIK not a bug but a technical limitation in OSX: Available slider controls (NSSlider / UISlider) on OSX don't natively support displaying tooltips like Windows does. Maybe at some point we can work around this via a custom implementation of slider controls, but that unfortunately can't happen over night.
Hope this helps.
When running Inquisit Lab on a Mac, I have been unable to get the slider control to show tooltips, regardless of the setting of "/showtooltips". Can anyone else confirm this as a bug?