The reorder by drag/drop works properly in 12x/19. Reordering by number can have issues.
Steps to replicate:
Add about 25 files/images to a sites resource tool
Select Actions->Reorder next to top level dir
On the top file, enter the highest number possible for the reorder(25 if you entered 25 files).
Repeat this for the next 4 or 5 items.
Scrolling to the bottom of the list, you will see those items are at the bottom as they should be.
Now scroll back to the top and on the top file listed, enter 10.
When you click out to save, it will move this file to the 3rd position.
This repeats when you update the top file trying to move it to 10.