An easier workaround would be simply to wrap the ConfirmButtonExtender in a <div style="display:none">. For example:
The problem in the first place is that the confirm button extender renders a new hidden button on the page (<input>) and the form decorator tries to decorate it. Since the new button is hidden, the style is not applied properly and the browser still renders a part of it (the ]). The workaround in this case is to wrap the button with a hidden div.
All the best,
the Telerik team