I checked in on the Apple discussion about the problem, and no mention of a general fix yet. Someone did post the form that you use to ask Apple to fix things. It’s here:
Meanwhile, my work Mac is running macOS 15.5, and my own Mac is on something from the future. The future version did originally also not have thumbnails, but it does currently. That might suggest there be some hope that the issue could be fixed in macOS 15.
One neat Mac feature is that you can mouse outside of your computer’s screen, and the cursor appears in the screen of another Mac (that is signed in with the same account). I can drag SketchUp files, that have thumbnails, from my own Mac into my work Mac. Before I drop the files I still see the thumbnails. After I drop them, they have a plain white document thumbnail. If I drag the file back to my Mac, they get the thumbnail back.