Thank you for bringing this problem to our attention - I reproduced and logged it in our database as a feature request.
The reason for this behavior is that we hide the original buttons, but the tooltip property is not copied. We will improve the behavior of the control in one of the following updates - I verified that only the decorated buttons have this problem (see attached video). We are also planning to provide styling for the textbox and textarea controls.
Your points were updated.
the Telerik team