In general, the root user has no special access to network shares, so if you try to use root as a login it will just be given the same access as any anonymous user. Notice when you try to setup security for any share that root is not one of the users you can select.
For this specific example, what do you have for Settings - Unassigned Devices - SMB Security?
If you disable SMB Security your Unassigned Devices will have public access.
If you enable SMB Security your Unassigned Devices will have whatever access you give to whatever users you give them to, but Windows won't actually let you change logins to network shares easily, even though it may give you a login prompt. See this post.