I have an up-to-date instance and tried to drag top menu items around and can’t. I ended up deleting them all and inserting them in the order that I wanted them. I thought it might just be me, but it got reported again.
So I’m thinking it’s a . (Discourse 1.9.0.beta14, Chrome on Linux).
If you want to rearrange things now it’s just a major pain. It wouldn’t be so bad if you could type between the existing options, or if you could just edit the whole thing as a comma separated setting.