Squoglybob, what sram chain are you using I may swap the Ultegra 6800 for one (mine is on the right way, I think lol)
I’ve the same problem as the op, rides fine in all gear, pedal back and it chucks the chain down three cogs, if you drop it down one from 42 it’s fine.
Off to the Apls on Friday and could do without the hassle but hopefully I wine need the 42 much.