I have the following options in smb.conf:
log level = 1
syslog = 4
syslog only = yes
smbd respect these, logging to syslog after minimal output to log.smbd.
nmbd does not, and continues to log to log.nmbd.
this bug is actually still valid with 4.18. However the syslog only parameter is deprecated and you should use "logging = file" instead of syslog only = yes these days. With that setting, logging to syslog will not be done. I'm closing this as WONTFIX now. The actual fix will be the removal of the syslog only parameter later on.