Has anyone else encountered the bug in the fourth level (Assault, near the harbor) where the enemy helicopter doesn't seem to spawn properly at the end? I saw a green dot on the map, but there's nothing there. The objective cannot be completed as a result.

I tried the level again and it looks like the enemy helicopter spawns at a very high altitude and it's almost impossible to see. I had to use 3 missiles with tracking to finally get it.

Only bug I've encountered is during the final battle. I fell through the floor and out of the "world".
I just restarted the level like you did and it worked this time.

I know this is two years ago but I've also noticed other vehicles are invisible, even when I'm in them.

It's not game breaking but it's a tad disorienting. :P

Same. So, is it a bug or rather a not so good design?

However, I'll try your solution.

How did you do? The heli is high behind the clouds, it's so far it's impossible to hit with the rockets.
I could hit it with the snipe (and I saw the balistics! the hit is not immediate at this distance), but once hit the heli moves just at our vertical, and as we have an angle (we aim up to 11:55) I no longer can aim it, strictly at 12 just above my head.

And, by the way, there is actually a bug: I see a plane, with fire spitting reactor, immobilized at 100m in the air.

A bug. The last helicopter is supposed to show up over the harbor almost immediately after the Korean cruiser is destroyed and be about as high up as the previous one.

I have seen it climb all the way into the skybox a couple of times too, but - if I recall correctly - the game kindly let me finish the level without shooting that heli down, even though this objective is green like it's mandatory.