Wrong polygon fill

Hello Guys,

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)

Examples:

Balaton
Geneva

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
add-pois-to-areas
add-pois-to-lines
adjust-turn-headings
bounds: …/bounds-latest.zip
check-roundabout-flares
check-roundabouts
copyright-message: Map data © Openstreetmap.org
drive-on=detect,right
generate-sea: land-tag=natural=background
gmapi
gmapsupp
housenumbers
ignore-maxspeeds
index
keep-going
latin1
link-pois-to-ways
location-autofill: is_in,nearest
make-opposite-cycleways
merge-lines
name-tag-list: int_name,name:en,name:hu,name,place_name
#nsis
order-by-decreasing-area
precomp-sea: …/sea-latest.zip
preserve-element-order
process-destination
process-exits
reduce-point-density-polygon=8
reduce-point-density=3
remove-ovm-work-files
remove-short-arcs
road-name-pois
route

show-profiles: 1

style-file: …/styles
style: default
tdbfile
x-split-name-index
verbose

Do you have any hints on that?

Update: if I get the lake in one piece from overpass, the wrong fill is gone.

There must be something with merging the separate OSM parts!!!