I think there might be a similar issue with the RadMaskedTextBox on Windows 10 Phone as well in IE.  We have it on a form, and it seems to work in all browsers just fine, although if I type fast enough in Microsoft Edge on a PC, it jumps around, and on a Windows Phone (Nokie 950 XL, running Windows 10 Mobile) where it moves the cursor as you try to type, but the numbers never show up in the text box.  A Required Field Validation on the RadMaskedTextBox also fails validation, although the cursor moved the entire length of every character entered.

Confirmed with RadControls version 2014.2.618.40 and Windows Phone 8.1

Works (mostly) with Safari on iOS 6 - here, the first character disappears, but at least the user is able to enter the remaining characters.

This was not an issue with RadControls version 2013.1.417.40 - the RadMaskedTextBox works just fine there, tested with Windows Phone 8.1