You should roll to the left and land safely, albiet requiring urgent fleeing motions to avoid become Scrab Chow. It sounds like you're just cursed. I don't know for certain whether you holding the move button down or not is enough to change anything, but sometimes when I've finished rescuing the last 3 Mudokons in Escape from RuptureFarms, I roll to grab the ledge and lower myself down, and end up not grabbing it and falling to my death, so presumably sometimes there is something to it. Experiment - try just tapping the move button when you've at the very edge of the ledge/tunnel, then try getting a good 'roll up' (run up) and seeing if that works. That's all I can suggest, except possibly just keep doing it over and over, shouting obsenities at the TV until it gets the point and stops mucking about with you.
