Depending on columns, thumbnails, etc., Opus may be requesting information that Explorer isn't. Or Explorer might be using a separate process which it closes afterwards and might hide the leak by the underlying codec/splitter.
You could try making a Process Monitor log if you want to get to the bottom of things. That will show not just which process is opening and closing the files, but which DLLs are involved in doing it.
(If you do that, tell us which file couldn't be deleted in the log, so we know what to filter on.)