Playing with parts in the workshop today and found Weird stuff.
New XTR di2 rear derailleur is compatible with ultegra 12spd DI2 shifters.
But neither Ultegra or GRX (825) 12spd DI2 rear derailleurs are compatible with the XTR 12spd shifter.
Strange.
That doesn’t sound right - I thought all the latest 12spd Di2 was inter-relatable? More than happy to be proven wrong and learn from it, though!
Calling @BetterShifting_Terry
Correct… that’s just the way it is 
A friend tried before launch, and yeah… doesn’t work
It matches the info in the compatibility charts, so they are correct for once 
Looks like I will need the SW-M8150 and run it it wired to do what I want.