Created a new form, placed an UltraNumericEditor.  Set the following values in the designer:

MaskInput = "-{double:3.2}";
MaxValue = 999.99D;
MinValue = -999.99D;
NumericType = Infragistics.Win.UltraWinEditors.NumericType.Double;
SpinButtonDisplayStyle = Infragistics.Win.ButtonDisplayStyle.Always;
SpinIncrement = 0.1D;

If I input 999.99 (max value), then use the spin button to decrement by a tenth I properly get 999.89.  Clicking to increment once puts me back at 999.99.  If I input 999.90 and increment once I get 999.99 (again, as expected).

Now input -999.99.  Incrementing with the spin button should give me -999.89, however it results in -999.00.  Attempting to decrement from here results in no change, -999.00 is incorrectly being enforced as the min value.  If I start at -999.88 and decement once I correctly get -999.98.  However decrementing from here I then get -999.00.

Latest service release of 11.1.