The described behavior is not expected and we have logged it as a bug in our Feedback Portal
where you can track its progress and vote for it.
The issue is reproducible when you set the Mask property to NoMask (Mask=""), therefore I can suggest you as a workaround to use another MaskToken
(e.g. #, Mask="#9") if this is suitable for your scenario.
Please excuse us for the inconvenience caused.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items