I've found this in version 18.3, but there is a high probability that this bug is still there in version 18.4.
In my game North America signed a pact with the aliens, and a couple of months later the aliens launched a terror mission in a north american city.
What's the use of alien pacts if they do not stop terror missions?
Here is the screenshot.
By the way, I completely ignored the terror mission.
If you sign a pact with alien forces, then do not ask for help...