I am brewing custom Garmin for my ZUMO660/396, but I noticed 2 parts (and there can be more) where the wrong polys are filled. Lake Balaton and Lake Geneva have land filled polys in it. (devices and basecamp also)
I am getting the OSM data from the overpass api in boxes of 0.5x0.5 degrees. Splitting in if necessary and using latest mkgmap to build the map.
mkgmap options I use (already tried various and does not affects the polys)
levels: 0:24, 1:22, 2:20, 3:18, 4:16, 5:14, 6:12, 7:10
copyright-message: Map data © Openstreetmap.org
Do you have any hints on that?