UPDATE: Got GOG version of the game and it works just fine. Looks like they fixed this problem in this version.
So yeah, as the title of this post says, there is a bug on one of the levels that prevented me from beating this game back then and these days too. And because of this bug I was unable to beat this game. Like, it happens in one of the mission where you walk near the buildings on some backyard and in one mission, when you spawn on that backyard, you will just fall through textures right away, and then die after few seconds. I have googled this problem but I can't find the solution for it. So yeah, because of this bug this game remains unbeatable for me even now.