When the door opens for the survivors to get in when they cap the 2nd CP, or did you do that and teleport to the vents, when I tried it I just hit an invisible ceiling like any other.
Well, I'd have to say that I'm not sure what "early" means in this content:
Alright I've tried this on Lab and Lab_remake, neither work and there's an invisible barrier than prevents you from climbing the elevator early.
Zombies aren't able to enter the elevator shaft before the second point is captured. Once red team captures that the door opens up for anyone to use, including zombies.
I think this is the issue here, once the door is open any player can either rocket/sticky jump or a zombie can leap up the shaft and that will kill everyone below.