I realized the following code returns U_USING_DEFAULT_WARNING in the return status code.
Calendar constructor initializes week data loded from locale bundles. In many locales, these week data comes from the root bundle. When accessing the data loaded from the root.res, U_USING_DEFAULT_WARNING is set. But I think this is not a good behavior. Of course, the internal API used here - ures_getByKeyWithFallback - is doing the right thing (at least, its comment clearly describe this behavior - U_USING_DEFAULT_WARNING for this case). But I do not think the status should be propageted to upper level for this code path.