This was first pointed out to me by a friend who jumped while downelevatoring on the first "Shrouded Bunker" level. I didn't have any saves that would allow me to verify, but on the second "Shrouded Bunker" level, when the elevator was moving down, I saved the game and was able to reproduce the issue that he described.
There were three outcomes from jumping while on a downward-moving elevator
- success: I suffered damage
- failure: I bounced up and down until I died
- failure: I was suspended in midair (I unable to consistently reproduce this failure mode, but when suspended I performed some action (not sure which, sorry) that caused me to fall and die)