The winding rule of the object is set to "Winding" instead of "Alternating".
There are a few methods to fix this problem. Reversing the filled hole subpath will make the hole non filled. An alternative method is to run the following macro with the object selected.
Dim s As Shape
For Each s In ActiveSelection.Shapes
If s.Type = cdrCurveShape Then
If s.FillMode = cdrFillWinding Then
s.FillMode = cdrFillAlternate
s.FillMode = cdrFillWinding
How did the winding rule set to "Winding"?
Most likely, one of the objects which were combined was originally a text object which was converted to curves. Some fonts have overlapping characters (e.g. a script font). When text is converted to curves the winding rule is set so that holes do not appear on the overlapping section.