I've been having issues with the game sort of lagging up rather than doing a stealth backstab using the knife. I'm honestly not sure the best way to do a backstab, but what I do is sneak up behind while crouched, then press and hold RMB to bring up hands, press and hold Shift to do heavy modifier, and then press and hold LMB to do the backstab. I've fallen through the ground only once, fell very far into water, from which I reconnected and was returned to ground.
Many other times though, I have tried to backstab and instead had a stuttering, where it is like I lose control of the camera for a bit, then don't perform the stealth kill.