The reason it drops on the inside is because the chain is trying to jump from small to large but having trouble catching on, and if the pedal stroke isn't smooth, or if the road buzz is causing the chain to jump around, it'll fall back down towards the small ring due to the chain tension, but it slips off the small ring for some reason. I've had this happen on mechanical FDs of the cable tension isn't high enough or the FD high limit is not set far enough, such that the throw of the front shifter isn't quite enough to help the chain make the jump onto the large ring. No experience with Di2 (eg, can you adjust the throw) but it could be just a case of needing to set the high limit correctly.