I fitted the same set up as chipbutty a couple of days ago, all new parts and new chain.
Mine drops two sprockets while back pedalling.
I tried adjusting the b screw despite not seeing how that could make any difference, which it did not, but I found if it was not wound in enough the chain would drop two sprockets then jam between the mech and cassette. I then tried using the supplied longer b screw and found if the b screw was screwed in too far it also jammed between the mech and cassette after dropping two sprockets.
So went back to the original screw screwed all the way in which was where I started.
The chain still dropped two sprockets but did not jam and climbed easily and quickly the moment you pedal forward.
I think this is due to tight tolerances in the new components and the narrow wide chainring which has very tight tolerances by design.
I really can't see it being a problem as I don't pedal backwoods accept for maybe when coasting behind some one slower while waiting for them to give way and defiantly not in a situation where I would be in such a low gear.
And if turning half a turn for pedal positioning I would always pedal forward.
I also think as the drivetrain wears this will sort itself out anyway.