Better not flick, you never know what those services do with metadata. There’s lots of ways, if you don’t use any of the usual suspects (google drive, dropbox, …) there’s also non-registration services, e.g. https://wetransfer.com/.
However overlapping thumbnails definitely does not look like a metadata issue. If you haven’t done that already, do screenshots and run in debug mode (I think on windows there’s something included for that, not sure) to capture logs and report them to the digikam bugtracker.