When we set it with QT pro, we can see it with Mac finder.
We noticed that our poster frame (set in catdv) does become the thumbnail when we put the clip in a web browser (we tried with Outlook One drive and we accessed it via safari... It's the right poster frame).
That would make you think it's something about how MAC OS is working. We can test it on Windows but my guess would be that it works fine in any Windows OS?