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I've got a minor issue with my drivetrain (3x9). If I'm riding the largest or middle chain ring and shift to any of the lowest (largest) sprockets on my cassette, the chain tries to drop down from the chainring onto the next one; I've got no issues riding the full range of the cassette in the granny ring. If I shift back down the cassette the chanin stays on the ring it's meant to be on.
Since I shouldn't be riding extreme gears it's not really a problem but I was just wondering what might be going on. It's behaved list this was transferred from another (identical geometry) frame and shifts without issue in the stand (it's load on the drivetrain that seems to trigger it).
The mech hanger seems fine and the chain length isn't a problem either (as far as I can tell) and the limit screws are correctly set (a new rear mech. didn't change anything either).
Sounds like your chainline's a bit wide, maybe shuffling BB spacers may help.
It's a 73mm wide bb shell (as on the previous bike) with a 73mm Octalink (yes I know I'm well out of date!) BB so the chainline should be fine (50mm). I did wonder about the mech. hanger but that looks fine.
As before, it's not a major issue but it would be nice to sort it out.
I seem to recall you're in the Cardiff area, if so, feel free to pop into Sunset and i'll put the hanger tool on it. Are the stays shorter on the new bike, this can alter the chain angle a little more in the extremes, maybe enough to cause an issue. Maybe a slightly shorter BB, if available, may help, my recent enforced change to a 50mm from 47.5mm chainline made for a noisier drivetrain in the lower rear gears. Also worth checking your B-tension (can be upset by a bent hanger), if your jockey is miles from the cassette, it's not going to control how the chain sits on it very well.
Chain line, not b tension or anything else other than possibly wear.
Gear cable stretch?