The glitch I posted above was also present in S26 and the initial release of C4D 2023, but has now been sorted out in the 2023.0.1 hot fix that was released a fortnight ago. It's no longer occurring for me, and in the release notes for the patch Maxon list the following viewport issue as being solved.
Fixed an issue with distortions in the viewport with multiple displays on Apple Silicon platform
Thank you Maxon!
