I have done some more testing and confirmed that some of our applications actually do use KeyControl with number fields and it is working.
One thing to check is that you have the proper datadict class attached to your field and inherit from an appropriate base class that will do the proper encode for the data type you are using. For example, the field that I tested with defined the Control member as a KeyControl, and inherited from Field_number, which will ensure the values get encoded as numbers. When I switched the datadict class to inherit from Field_string, I saw behavior similar to what you described.