What is it?

Do you see the same in File Explorer for those folders?

Those folder thumbnails are generated by Windows, and Windows 10 (since release) introduced a bug where they are sometimes not rendered properly.

(In my experience, it results in them having a solid black background sometimes, in both Opus and Explorer, but it may not always be the same on all machines.)

Folder thumbnails are normally cached when generated, and if they are not rendered properly then the incorrect result is cached by Windows until something causes the thumbnail to be regenerated. Adding a file to the folder the icon represents will usually trigger that. (Clearing the Windows thumbnail cache is another option, but since the problem is random it can simply move it to other folders' thumbnails.)