The observed issue is a specific browser behavior. In some cases the empty message can be filled by the browser inside the textbox. When the browser populates this message, we can't tell that is the empty text, since the design of the RadTextBox supports the input to have value the same as the empty message.
However, this issue is already forwarded to our Development team for further research and in case they find a proper solution for it, the fix will be included in some of our future releases.
I hope this helps.
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