Any update that includes a change in the /etc/smb.conf file can overwrite changes made inside the main configuration block between ; BEGIN and ; END.
Changes to the /etc/smb.conf file must be made after the ; END required configuration line to be preserved.
This situation is noted in the /etc/smb.conf header:
; Site-specific customizations will only be preserved if they are done
; outside this block. If you choose to make customizations, it is your
; own responsibility to verify that they work correctly with the supported
; configuration tools.