I have noticed something else.
This is not so much a bug, but a "weirdness".
The aliens seem to behave in a predictable manner.
Lets take the rome map. It is a rather difficult map, with two bridges, and a
very long area. You can sneak from house to house. Then, once you reached the
other side, the aliens will in like 80% of the cases, aggregate in one building.
You can kill them all one by one by moving outside your cover, shoot at them,
and move back again to cover. The aliens will in most situations never leave
this place.
There is more weirdness actually, the aliens will tend to aggregate in certain
hotspots of maps. I think in some time I will know all their pattern and choose
the best situation to snipe em
(that being said, i sometimes make mistakes and
an alien gets one of my guys... cover is really vital)