Di2 rear mech one gear out of phase.

That will only get it into adjustment mode, won't it? Or will spinning the pedals make it think it's a crash? I guess I can try it.

Hold it down for full 5 seconds not just the 2 seconds it takes to get it into adjustment mode

Adjustment mode is when the LED goes red, keep holding and it does a reset

But the fact it's changing gears now means it's not in CPM so I don't think that's the issue. I reckon something is bent the cage is wrong or the RD isn't on the hanger properly.

What's your biggest cassette cog size?

I've just got a 28 on there ATM. Normally have 40.

I know it's not in CPM, I want to get it into CPM to reset it.

I'll go and try this now, but I think I normally hold it for 5 secs to get into adj mode anyway.

EDIT- just tried it. You were of course right and that is what happened - it went through adjustment mode and did then the reset where it shifts through all the gears. But it is still the same :(

I can lend you an older Di2 RD that will work with that cassette then? Or do you just wanna test it?

Reset is reset. Doesn't matter if it's in or out of CPM to do it.

Just want to test it. It's an old mech I'm using, with a wolf tooth.
I could have bent the wolf tooth. Not checked that.

Yes - it was as you said! But didnt' fix it.

OK, another idea. I've unplugged it and will leave it, then reconnect in an hour and see what happens.

Successful diagnosis thanks to @hippy.
mech is fucked

Dibs 😁

Oops, I left the Mason mech at your place too. I'll need to come back and grab that.

OK, I'm just heading your way now, can I drop it off?

I'd be curious to know if my solution to go into adjustment mode on the 6th sprocket works, now that you've identified the problem :)

If the RD is bent such that even at full trim to the right it still can't get the next cog, I don't think it matters where you start the adjustment. I think I said up thread I've never bothered to choose the start cog specifically and it has always worked. I think the issue is the RD or the cage is bent too far inwards, even at full trim (although I didn't check that).

Isn't the cog at which you start the adjustment the one that is considered to be the fifth by the deraileur (don't have my bike at hand to test it out unfortunately)? In which case that could solve @frank9755's problem, because he seems to have an offset of one cog but fine shifting otherwise.

Your experience seem to indicate otherwise, but then I'm wondering how Shim hardcodes the position of the fifth sprocket among all available cassettes. I guess the distance between the hanger and the n-th cog of the cassette doesn't vary much amongst cassettes, if at all, but then I'd have let the user define the reference themselves.

You can only move the RD x clicks left of any cog and x clicks right of any cog. So the cog you start the adjustment doesn't really matter if you've got to max adjustment and it still won't get the next gear because the RD is bent.

Added some stuff to my previous answer while you typed yours :)

I hadn't realized this was the problem, I thought the gears changes were fine but @frank9755 couldn't get the 11T sprocket due to not having gear changes left, while having one "extra" gear change on the leftmost side of the cassette.

Alas not! I tried that right at the start. It just thought it was in the 5th one and acted accordingly.

No, it doesn't take into account where you start from. Shimano knows best and there's no way to override it!

Bummer, can't trick the megacorp then!

Yes that is the problem. I can't get the 11T and have an extra sprocket because there is something wrong with my mech.

It's not the hanger or the frame, or even the Wolf Tooth, as @hippy's spare mech gets all the gears when we put it on my bike.

Back in November I was riding along and got a weed caught in my chain. It went into the mech and pulled it hard, like an anchor, so the hanger snapped in half. Most likely it bent part of the mech. I had already replaced the plates as I thought they might be bent. It could be aonther part of the mech that is bent (but the old cage was hardly bent if at all and probably would have been mangled if it had been tweaked hard enought to bend another part of the mech, so maybe not?), or I might not have fitted the new plates right, or it could be the mech's software, or something else. But none of those seem to be fixable - by me at least, so I'll just get a new one.