Problem with SVG thumbnails. Strange behaviour. Sometimes it shows thumb sometimes not. When SVG with no thumb (associated program icon shows in such cases) is dragged and dropped into destination the thumb will show and when dragged back to source the thumb will still show but navigate out of and back into folder the thumb has been replaced with associated program icon.
Opus isn't involved in displaying/decoding SVG files in any way, at least currently. (Display of them in the viewer pane is via IE at the moment, and will switch to Edge in the future.)
Some other component on the system must be doing the thumbnailing. I think it's built into Windows or Edge these days, but I'm not sure.
You'll likely see the same thing in File Explorer.