I'm running LR Classic 11.5 on a Mac Studio M1 Max with 32GB RAM and a Mac Studio Display. This is a fresh install of LR on a new Mac using the library I copied over from my old iMac.
In the develop module all my photos display perfectly with no colour/compression banding. However if I go to the slideshow module than many, but not all, show mild to severe colour banding. I've set the quality to high but whilst it helped, it hasn't fixed the problem.
If I export any of the photos as jpegs, they all look absolutely fine.
I've tried enabling and disabling the use of graphics processing in preferences>performance but it made no difference.
Do you see the same banding in the Library module? It is probably jpeg compression banding or banding caused by color space conversion. The fact that changing the quality on the previews (catalog settings) helped indicates that this is probably the issue. There might not be a way around it therefore. I moved away from using Lightroom for slideshows because the slideshow quality is so bad for both sharpness and banding issues. Even simple built-in Mac Photos app does better. Just export a bunch of jpegs and import them in Photos and hit slideshow.
Yeah the banding is there in the Library module as well and it's definitely jpeg compression. The fact that exported jpegs look fine suggests that Adobe could quite easily fix the problem by changing the compression ratio but I guess they have their reasons.
It's a shame as the slideshow module is now unusable for me and I used to use that quite a lot to preview my selected photos so that final selections could be made for printing/export. Now I'll have to export them all first. Not the end of the world but an unneccassary extra step.