it's on all maps with totally destroyed UFOs (see the screens, haven't seen one of those maps without that bug), those non-solid obstacles are the main reason for invisible aliens on those maps.
I'll attach some screens... first screen: that alien was invincible in that spot, i couldn't even hit it with a stun rod or explosives. 100% hit chance, but still all missed.
screen 2&3: hidden alien -> move into it = visible.
The parts where they can hide in are the one to the right and the whole thing under which the dead civilian lies. Also when the aliens start their move in one of those tiles you won't see them move... and they
really like that. Found 4 aliens hiding in the same square once...
same goes for the above mentioned Tank-Things inside UFOs. For example on that africa map ( in south america
), see screen 4. (and i fear there is another one in the other room's inventory, too - so i won't be able to win this map
another things i saw aiming tip show wrong % chances 5/6 bursts at chance 0% i got perfect hit sometimes miss at 76-85 % well if it happen 1 or 2 times would be ok but it happen often
Those to-hit percentages aren't working at all, i regularly hit with 0% chances. With hight percentages it's almost fine, but low ones definitely aren't displayed right.