There are 3 lanes, and you can go under them on this map. Under the center lane, there are stairs that make you go near the ennemy spawns of the middle lane. If you go down those stairs, there is no problem. If you try to go upstairs, you just fall off the map, there is no collision here.
Expected Results: Just put a real staircase there.
I came across this bug today, too! It actually made me laugh, but definitely needs to be fixed! I can attest that going down the middle stairs is fine, but going up the stairs makes me fall in a pit. Happens 100% of the time.