Di2 rear mech one gear out of phase.

As a last resort you could post a picture of the derailleur as seen from the back of the bike, maybe somebody can identify if there's anything wrong with the way you mounted the replacement cages, but I assume you and @hippy have already checked that.

Always disappointing to replace these parts, broke the hydro bladder of my Di2 brifter after less than a 1000 km due to a bad fall, I was a bit upset.

I couldn't see anything obviously wrong with it, but it only has to be a subtle bend or twist of one part of it to throw shifting off. It could still be a useful spare, just not gonna win any sprints (because everyone is sprinting in the 11T trufax)

If the original hanger breakage came about because the mech took a hit is it possible that that hit has knocked something within the derailleur mechanism out of place? Like if there’s a gear that the motor turns in order to move the derailleur it’s skipped a few teeth and the derailleur position and motor position are now out of phase? The fact it can over shift into the spokes but not get to the smallest cog would (in my mind) support this theory.

It's a good as theory as any I've come up with!

If I thought it was always going to work exactly the same it would almost not be worth replacing as I only use that bike for ultras, audax, light touring, off-road gravel messing around, etc - and spinning out a bit sooner with no 11T just means I can roll down the hills longer. But the risk of failure mid-ride is most likely a lot higher now.

It is an unwanted expense!

TBH, though, if I had to break and replace one part, I'm glad it is that mech. About a year ago I was going up a crazy steep hill in bottom gear, standing up and hardly moving, and the cage give way and upshifted, and I fell off. I've never really been happy with it since and was actually thinking about replacing it after that.

I've had six years' use out of it so can't complain.

LBS suggested tweeting Shimano and asking about crash replacement policy. I expect they'll say no.

I'd save it for a gravel bike, turbo trainer bike, emergency spare, that kind of thing.

crash replacement policy

Please let us know if that's a thing. I'd love to send them all my broken RDs for discount on new ones! :)

Just tried this today. Madison evasive at best. Totally don't give a fuck at best.

We had this a few times. In some cases a file or a washer just do it, in others it's missing half of the cassette.
Annoying that only campagnolo can do a zero reset