What is supposed to happen is that combine will realize it can't server-side move (because the upstreams are different), and then fall back to regular move (copy + delete). There is a more detailed explanation here if you are interested.
Ok, very likely a VFS bug then. If it's possible to make a reproducible example, I can try to debug it.
I'm also wondering if it's specific to Dropbox? For example, does the same thing happen when trying to move from onedrive to pcloud?