to post #8 .

Also, mind the gap distance btwn the FD outer plate and big ring. Set to ~3mm and no more. Uploading a photo might save some guesswork here.

It is important to use the Di2 FD set screw too, and against a firm backing plate. The Di2 FD clamp from Shimano provides a very good base for the screw to hit. This screw helps augment the FD strength and keeps if from twisting under load.

Chain type shouldn't be an issue here as that typically is a non-factor in FD downshifts.