A SMB client with a username equal to the SMB share name cannot access that share. Logs show that TrueNAS is attempting to direct the access to a home directory for that user. The behavior is the same whether the user's home directory is set to /nonexistent or /home/[username], or a path to the dataset with the same name as the username.
I cannot attach the logs, but here is an exerpt from /var/log/samba4/log.smbd when the SMB client attempts to use credentials where the username matches the SMB share name. I have sanitized the username and replaced it with [username].
make_connection_snum: SMB_VFS_CONNECT for service '[username]' at '/mnt/zpool1/home/[username]' failed: No such file or directory
I seem to recall running into this problem in FreeNAS in 11.2 (I am not certain about the version number though) and worked around the issue at the time by changing the SMB share names. However since the middleware is looking for a user home even when the home is /nonexistent this appears to be a bug that should be resolved.