the photos aren’t disappearing exactly. they’re just not loading successfully. the next time this happens, you can look in your browser’s developer tools console, and you’ll probably see some error messages logged there that may provide more detail about exactly what failed during upload.
that said, there have been many previous discussions in the forum about workflows in Mac that seem to result in lots of problems. among the many such threads, here are a couple random ones that look like they might be worth looking at: