There is indeed a problem with the RadMaskedTextBox control which I notified our developement team about. It seems the control persists the string from the display mask, instead of the value of the control. Our developers will investigate it further, confirm whether it is a bug and if so, fix it.
In the meantime, you could as a workaround use identical Mask and DisplayMask values, in this scenario there is no such problem.
the Telerik team
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