your example with hardcoded values works as expected. But when you add a ViewModel and bind a control to a model property, then the missbehaviour appears.
I added a simple ViewModel class with a single property "Name" and defined the Tag/Html-Helpers as follow:
@(Html.Kendo().TextBoxFor(m => m.Name))
The HtmlHelper works fine, the TagHelper does not. Sadly i can not attach a .zip file here ("allowed: .jpg, .jpeg, .gif, .png"). Otherwise i would attach the modified project.
Thanks in advance.