Placeholder is set to " " and the SelectionOnFocus is set to CaretOnBeggining. When the Mask receives the focus, the caret is under the MouseClick position, not in beggining.
After load and clicking in the center of the MaskedInput , no value can be inserted.

The xaml team recently reviewed the status of this issue and won't be addressing it in future. 
Placeholder set to string.empty or ' ' should be considered not supported. Please consider using Mask="" (no-mask) instead.