I had to fuck around with an slx derailleur to get it to work with an 11/46, and shifting it shit. When I can be bothered I'll go back to 11/42.
I've the 1by di2 grx derailleur running as 2by with an 11/42 fine.
Yeah, 11-42 works with stock Di2 DRX and Ultegra shadow cage (1x RD w/ 30/46 or 34/46 rings).
I've never tried 11-46T though. Do you remember the chain length you used?
I had to fuck around with an slx derailleur to get it to work with an 11/46, and shifting it shit. When I can be bothered I'll go back to 11/42.
I've the 1by di2 grx derailleur running as 2by with an 11/42 fine.