From my understanding, the label comes from the multipolygon corresponding to the lake (relation 332617), but I can't see anything wrong with it.

And the label for "Lac de Neuchâtel" also seems oddly located, although it's at least in the lake, see this map:

http://www.openstreetmap.org/?mlat=46.9521&mlon=6.9598#map=10/46.9521/6.9598