For interest and information. Recently my friend's di2 11 speed front derailleur FD-R8050 failed. It started making a high pitched squealing and would not shift. It was 4+ years old so out of warranty.

I put it on the PC using SM-PCE02, and it failed the diagnostic check unsurprisingly. I decided to investigate, and with a bit of help from a YT video, got it apart. I discovered rust around the linkage arm, and it was really stiff. Running it with the cover off showed the drive not transmitting to the crank.

On closer inspection, I observed that one pinion (about 3mm dia) had worn teeth, which was the reason for the loss of drive. I'm pretty sure that the excess load from the stiff linkage arm was the root cause of the failure. He's got a new one now.

Learning is to lubricate the crank/linkage arm frequently (WD40, GT85) especially after a wet ride.