Felix,
Just another thought. Are you using single color or pattern for land? Try using a pattern for land (set both colors the same, or for maptk set just one pixel to a different color).
Using single color instead of pattern, does definitely sometimes lead to problems. Garmin modern tpyfiles use patterns for all polys.
OK - noted. You will be pleased to know that I have taken your advice and started looking at using MapTk to generate the typ file. I can generate a file but, weirdly, mapsource is ignoring my polyline styles. The couple of POIs I have and the polygons appear to be working OK, it's just the polylines it is ignoring. The file isn't huge so I have attached it to this email, please take a quick look at the polylines in case there is something obvious I am doing wrong. BTW - I can edit those polyline styles in MapTk and they look just as they should. Mark