In the latest update (1.7.1) a new room type was introduced. This room looks like a regular locked room (like a “laboratory” for example), so a player was expected to search the level for an iron key. After failing to obtain it, the player had to find another way to get inside. This new room is quite different from the rest of the “special” rooms in the game and I was ready to receive a lot of complaints about the game being bugged. I was going to answer something like "No, it’s not a bug, it’s a kind of riddle and it’s not that difficult".
But actually there was a bug, and one, but many. And as a result many players faced “unsolvable” level layouts, I’m sorry about that. I have released two hotfixes and I hope that all issues with missing keys are resolved now, but I’m not 100% sure.
So here is the answer for the puzzle: the key for the door is inside of the room and to get inside you need to jump into the chasm from the level above the level with that locked room. There is another special room (so there are totally 2 new rooms types in the game), that looks like this:
The “leap of faith” has to be performed in this very room, otherwise you’ll land not in the locked room, but in some random location.