What have I done wrong?

Nothing. Mine does that.

Freddie.
Have you used a new chain?? I have a theory.

My partner has a 4 ride old chain and a t-rex and it doesn’t drop.
I have a new cassette, new jockey wheels, new chain, new front ring.
Drops when back peddling. I’ve used it a few times and the drops are less.

My assumption is the chain is so new it can’t flex to the chainline and therefore drops when back peddling. As the chain has worn over the few rides I can back peddle more and more with less dropps

I had that issue with the Oneup cassette ring but it turned out to be chainline rather than chain as I had converted an XTR M980 from 3X10 to 1X10. Swapped to an M980 2X10 with a 3.2mm less chainline and problem solved.

I know my chainline is dead on running 51mm with a next sl crank on a single ring with a small spacer on the drive side.

FWIW my OneUp doesn’t do it.

Mine is running a brand spankin new XT cassette and SLX chain. Could be tha it’s new.

My kit is all new, so I could be that I guess.

Mine does it too, going to get some fixie spacer washers to improve my chain line.