The only way I was able to fix that was by uploading the primary preview first and then the rest, but even this tactic did not help me when I was trying to edit a product the last time. Had to republish as new model.
I have requested that the upload system would be fixed by forcing the image upload by name instead of randomly so that the images would be in proper order after upload - no need to rearrange. This was 3-4 years ago an nothing has been done about it (including the product edit issues).
At this point I can't even access the editing page on some models as I get 404 error. Reported it a week ago and still not fixed.