Assuming Mystic Thumbs installs a 64-bit IThumbnailProvider, it should work in Opus the same as in File Explorer.
People have mentioned using it with Opus a few times over the years. There was one report of an update to it breaking things but I don't know if that went anywhere or if it is still relevant (See the thread: Mystic Thumbs).
That means Opus is asking the Windows shell if it can generate thumbnails for those files. It should in turn ask shell extensions like the Mystic Thumbs one to generate thumbs if needed. Once a thumb has been generated (and if it took more than a very short amount of time), the shell will cache the thumbnail for speed. Asking the shell again after that will just retrieve the cached thumbnail.
So it sounds like for some reason Mystic Thumbs thumbnail generation is working in File Explorer and then being cached, and the cached copy is then available to Opus.
I've seen that happen when only a 32-bit thumbnailer is installed, and not a 64-bit one. If that is what's happening, you'd only see thumbnails generated if a 32-bit process was doing it. File Explorer is normally 64-bit, but if you're causing the thumbnails to be generated via a File Open dialog, from a 32-bit program, then that could well be what's happening.