Since installing Sonoma, Preview says "The document ... could not be saved" after editing.

Cerastez, are you saying that Apple responded to your feedback, and they offered that explanation? If so, it's slightly problematic: It appears that when you select "true grayscale" from the Brother iPrint&Scan, it yields sRGB IEC61966-2.1, which is indeed 8 bit, but that profile is apparently not exclusively grayscale -- though I suspect that's its primary usage.

More importantly, dropping support for a color profile, without any warning or explanation, is most certainly not a "feature". This is an extremely useful color profile, allowing you to save a lot of storage when archiving documents.

Perhaps you didn't really mean that it was a feature. It was a (presumably) intentional deprecation, which is something else entirely.

ArchonOSX wrote:

I had updated to Sonoma and now un-updated back to Ventura. Sonoma was a pretty big catastrophe for my use case so I am back to Ventura.

That being said, Preview in Ventura works just fine and I can now rotate and flip 16 bit gray photos with no issues.

Not going to update systems again. Everything from now on will be a clean new install.

Happy Day!

Just keep in mind that your approach is only a temporary workaround. Eventually (in a year or two?), Apple will drop support for Ventura, and your system will become increasingly vulnerable. And, of course, eventually, updates to any third-party apps may become unavailable as their minimum OS version requires Sonoma or whatever comes next.