Hey Rob,
After experimenting and investigating for some time, we have finally found a definite way to reproduce the issue. If I'm not mistaken, after uploading the images, wait a while, then press the submit button.
The reason the images do not exist is because the data from the table #__social_uploader is deleted before the submit button is pressed. Now this does not happen immediately after the image is uploaded, which is why I mentioned waiting a while before pressing submit.
https://take.ms/ibWGS
However, we are not sure what is deleting the data in that table. In our local instance, the data is not deleted until after we press the submit button. Do you recall or know of any php script doing this? Or if you have some other suspicions, let us know.