Indeed, there is a problem with this property for the masked textbox control.
I have logged it into our system, and alerted our developers. They will soon address this issue.
Additionally, I have updated your Telerik points as a token of gratitude, for bringing this discrepancy to our attention.
I just tested the functionality and it works as expected with our latest release. Note that in order for the EmptyMessage to be displayed for the RadMaskedTextBox you need to set the HideOnBlur property to True as explained in this help article.
Sincerely yours,
Pavel
the Telerik team
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