Check out this save - my guys are going to a landed medium ufo mission right next to an alien terror attack, the thing is that after i successfully finish this mission i get several messages that there is a terror attack (i click on "center on event and set time to 5-seconds" button), one immediately after another, and then i get a message that terror attack is successfully thwarted. I reproduced this bug three times already with different terror (or is it "panic"?) missions and landed medium ufos. Is there a reason they all land next to a terror missions or is it just me being lucky?
This is the save from ground combat just finish couple of aliens in the ufo and then check out those messages on Geoscape.