Drag and Drop is not working correctly

I have a project (P-17015) containing several milestones and a sub-project.  The sub-project was arranged below the list of milestones, but today I found a couple of the milestones appearing below the sub-project.  I tried dragging the milestones above the sub-project, and it appeared to work.  But refreshing the screen put the milestones back where they were before I dragged them.  So then i tried dragging the sub-project to the bottom of the list, which again appeared to work.  But after refreshing, the sub-project appeared even higher in the list of milestones than it was before.

How can I rearrange the listing?