I logged both broken multipolygons (303344 and 302922) with logging-level FINE and seperated the details of both relations. You can find the loggs at http://hscholland.de/data/mkgmap.log.7z
aighes
Ok, things are clear now. The multipolygon was tagged only with name=Vanajavesi and the three outer ways were tagged with natural=water. Since r1711 mkgmap generally uses the tags from the multipolygon if it has at least one tag (type=multipolygon does not count). So the mp (which means the merged polgons) is tagged only with name=Vanajavesi. The tags of the outer ways are not removed so they still use natural=water. This leads to the described effect as they are autoclosed (I don't know where this happens). Correct tagging is now necessary for a correct processing. Before r1711 the tags were taken only from the multipolygon if it had at least one known "polygon"-tag (like natural, landuse etc.). But there was the problem that the list of known polygon tags could not be complete. So we decided to require a clean tagging. WanMil