Can't believe I'd not realized this before - but I guess I had been doing most of my work with these files from a USB drive, and have lately done more with files inside one of my Libraries...
A zip file that contains all JPG images that Opus otherwise properly shows thumbnails for when browsing inside the zip file if the file is located under a physical drive path - shows only zip folder icons for as the thumbnail for each JPG when the zip file is located under a virtual library path.