Doing a split produces invalid files with incorrect root elements. The root element should be oval_definitions, but instead it is the element from the level where the split was done. For example, splitting at the definitions level results in a root element of definition. Splitting at the tests level produces a variety of root elements, matching whatever is used in that test (file_test, registry_test, etc.). Only the root element is incorrect - everything below the root element is what you would expect to see as children of the oval_definition element in a proper OVAL file, starting with the generator element.
Sample of bad root element on definition split