Win10
Mod
This soldier got stuck in a "wall" after overwatch.
I tried to move others next to his location, and I was told that he is a high cover.
I think the game though he was in a high cover object which I destroyed many turns ago.
Seems like a vanilla bug that enemies can take invisible covers, but I think that bug has been fixed.
I also noticed that soldiers with rapid reaction or sentinel skill will eventually step out their covers graphically after many overwatch shots.
And game got frozen again when I reached the relay.
Pretty much the same thing in this post.
Will try to do it again later.