I just attempted a mission where the aliens could use psionic attacks – very unbalanced, they could attack every member of the team even without any visible…
According to the wiki:
“It should be noted that once you pass Turn 20, or if there are only 1 or 2 aliens left on the map, all of your squad members and their locations are revealed to the AI. This means you can expect a steady stream of psionic attacks after that turn if there any of the remaining aliens are capable of Psionic attack.”
Apparently the only defence is just getting rid of the sucessfuly attacked soldiers.