+1 to moving cranks forward and back when off road, sometimes it’s just easier to spin the crank back a fraction to re-adjust position.
For the chain coming off, I’ve never had it on any standard cassette. I have had it after putting a 40T Hope Expander on. The chain drops pretty quickly, certainly within a single turn of the cranks.
It does seem to be a chainline thing, what I don’t get it whether it’s the bigger size of the 40T that brings the chain down or whether the ramps on the cassette normally help retain the chain and it’s the smooth teeth of the 40T that’s a problem.
It’s not that huge a problem as if I’m using the 40T it’s because I’m blowing out of my arse trying to struggle up a hill and moving the cranks backwards is the last thing on my mind, I’m normally 2 or 3 gears down the cassette when starting off.