http://www.openstreetmap.org/browse/relation/6128
http://www.openstreetmap.org/browse/relation/58910

these 2 relations describe a forest in germany, the forest being the outer polygon, clearings, a quarry and so on the inner ones. the relation istself has (actually had, i corrected it just now) only type=multipolygon and a name. mkgmap renders all the inner polygons, but only some (randomly, can't find a significant difference) are cut out of the surrounding forest.

as soon as the relation has a landuse=forest, everything works as expected.