There was definitely an Ember regression here with SortableMixin which is no longer present. I’ve fixed it here:
https://github.com/discourse/discourse/commit/69ff0e48b4b50ad3df78dcb4f52915eef6850de9
Having said that, I’m not 100% sure that fixes the problem in the OP. I found the interface a little confusing, but when I reordered categories, I first had to hit “Fix Positions” before I could hit “Save Order” which was unintuitive. Once I did that though, things seemed to save?