Looking at the definition for addr:flats, I think possibly the number range belongs on doors (ie, on one or more nodes on the perimeter of the building tagged entrance=main) rather than on the building. An example or more specific instructions would be useful, but entrance is mentioned under the "useful combinations" and building is not. So my guess is that's why the flat numbers are not being rendered in mapnik. But to test that theory properly, and to fix it if appropriate, we would need to know where the entrance(s) are and whether they cover a limited subset of the overall range 