Fabricio,
If you zoom the upper left of your polyline, the one on the right is actually self-crossing,
so I doubt that it is correct.
You can also see this if you Pedit then edit xertex and then follow the vertices.
Still the problem with the area remains. It all has to do with the extra vertex that is created by
simply changing the property to closed.
If you actually remove that extra vertex your area is OK.
ymg