IgnacioA have found that previews on Quick Look no longer work, because QL generator plugins are deprecated and this new OS is removing dropping their support.
Good catch, and appreciate the heads-up: Yes, macOS 15 "Sequoia" will remove Quick Look support and apps have to use Preview / Thumbnail extensions to provide content previews.
Apple's documentation states in Sequoia Beta 3 Release Notes: "Support for deprecated Quick Look Generator plugins is being removed. To provide previews and thumbnails for your custom file types, migrate to Quick Look Preview Extension and Thumbnail Extension API. (116791365)".
StitchBuddy HD on iOS is already using these APIs, and I will ensure a smooth transition on macOS by adjusting my priorities. Thanks again 🙂