Indeed this is a bug in our MaskedInputControls. We logged it in our feedback portal
where you can track its status. We also updated your telerik account points.
Just for clarification - this is not regression bug, this scenario has never worked in our previous Releases.
The fix will include the following - Pasting "A" next to "ABC" will produce "ABCA".
As a workaround if this is applicable in your scenario, you can set InputBehavior="Insert". Other workaround is to override RadMaskedTextInput and its method HandlePasteNoMask.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.