We logged this issue in our PITS and you can use the following PITS item ID to track its progress - 2872. What I could suggest you to overcome this is to use your approach is to handle the PrevewMousDown but only if the OriginalSource of the routed event is within the MaskedTextBox control and not within the DropDownButton of the Calendar.
Hope this helps!
All the best,
the Telerik team
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