Ever since 1.9 came out, when I save Designer files to iCloud Drive, they don't show custom thumbnails. They only show generic Designer thumbnails (screenshot attached).
When I sort by date in Files.app, there's a clear switch from when documents used to get custom thumbnails and when they stopped.
If I send the files to my computer, I can see thumbnails in Finder and Quick Look works as expected.
Most of the Designer work is done on iPad, though, and I'd really love to figure out how to get thumbnails back. Because File.app shows thumbnails from previous versions, it doesn't feel like the whole system is broken, just for new files.
