Postgis intersection behaves strangely when getting closer to poles

We use Postgis for Django to locate the objects inside a certain viewport. It works great at the equator, but the farther we move, the more distorted the viewport becomes.

Here is the result (the red line shows the "ignored area"):

Same on the southern hemisphere:

We retrieve the viewport from Maps, build a Polygon and intersect it with the coordinates of the objects:

viewport = Polygon.from_bbox((southeast_lng, southeast_lat, northwest_lng, northwest_lat,))
result = queryset.filter(coordinates__intersects=viewport)

The coordinates field in the object are defined as following:

coordinates = models.PointField(_('Coordinates'), srid=4326, null=True, geography=True)

Additional info:

• The "missed area" depends on zoom level and vicinity to the pole
• Setting geography=False fixes it! But then we have problems at the 180th meridian.

Does anyone know what might be causing this?