What do orange brackets at the 4 corners of a preview tile mean?

What do orange brackets at the 4 corners of a preview tile mean?

Since this last release, I'm seeing orange brackets at the 4 corners of preview tiles for *some* images from my Fujifilm camera. I can't find anything in the documentation on what this signifies. My first guess was that high res previews weren't generated for those images because they're from a Capture One catalog, but the brackets down't go away after generating high rest thumbnails via the interface.