So after a lot of time spent on chat and phone with Adobe. It seems that I am unable to use v25.4.1 or v26 in GPU mode unless I upgrade to Monterey.

However Monterey is in its infancy. I'm not taking the risk of upgrading yet.

My machines both have the same GPU redraw issues, where images within clipping masks create echoed glitches of themselves across the screen. this disappears when GPU mode is turned off.

CPU mode in Illustrator is so sluggish it's pretty much unworkable.

Adobe informs me this is NOT an Adobe issue, it is an issue with my graphics card and it needs to be updated. however, on a Mac, the only way I can do this is to move to Monterey.

It seems the only solution is to revert back to v25, which works.

MacBook Pro 15" 2018

Graphics card -
Radeon Pro Vega 20 4 GB
Intel UHD Graphics 630 1536 MB

Imac Pro 2017

Graphics Card -
Radeon Pro Vega 64 16 GB