The SpinMode property is set to Position and the Mask is like "d" or "#".

 The "d" means that a digit pattern is required , "#" is for non-required digit. 

When the caret is on the empty placeholder and the Key.Up Arrow is pressed, non-numeric characters appear first ("! " then """ then "#" etc).

The expected behavior is that only digits are looped in such mask *digit* positions.