I'm not sure if this is 100% a Windows 10 bug or if FreeNAS also has some play in it, but I do want to notify you so you can figure it out...
When you try to "map a drive" a FreeNAS SMB share on Windows 10 with the name testshare and select "Use different credentials" and login with testuser, it works after filling the credentials twice (not sure if that is already the first bug?).
After this, when you try to "map a drive" a FreeNAS SMB share on Windows 10 with the name testshare2 and select "Use different credentials" and login with testuser2 (so different credentials)), it doesn't work (see attached screenshot for the error) because Windows thinks the drive is still mapped with different credentials (but it is not, only testshare is still mapped).
After disconnecting testshare, it is possible to map testshare2...
See the attached debug for details on how to reproduce the issue.
This also works with for example tshare3/tuser3 and tshare31/tuser31
See here for more details / screenshots: