New cassette: 11-32 to 11-25......Di2 now not shifting correctly. Any ideas?

My bike's stock componentry was Ultegra Di2 6800 (compact 11-32 / 50-34)

Last month installed new semi-compact R8000. Chain left as was. Smooth shifting as per usual.  Very happy. All good.

Two days ago, I ditch the Ultegra cassette/chain and add a shiny new Dura Ace C9000 11-25 cassette and a KMC X11EL (nothing out of the ordinary) - but ever since, the shifting is all out...ESPECIALLY from 11 to 10

It actually shifts kind of OK between 9 - 1 but no where near as crisp as it was before. No amount of micro shifting does the trick. Basically, if you get it perfect in one area (near enough impossible) it goes out in another.

For the KMC chain, I have kept the same chain length again - though it was slightly shorter due to old one having 1cm stretch - and visually it looks OK....but I can't be 100%

Also, mech hanger looks OK and is unlikely to have been damaged during work being carried out....but again can't be 100%

So, I'm confused: too long a rear mech cage... chain compatibility/length... alignment issues...bent hanger or bent cage.....or what?!

Any ideas?

Thanks