You have the following scenario.
The UpdateValueEvent property is set to "LostFocus" and set no mask ( Mask= "").

When the input box is focused, press the minus sign key. You can see the that the minus is showing as expected. 

Then press a number and the "-" (minus) disappear. By pressing number key again the minus appears again.

A workaround: 
Setting the UpdateValueEvent property to PropertyChanged


Available in R2 2016 SP