See this bug. Calamares version 3.2.35 or greater is needed and settings in calamares-settings-ubuntu need to change.
In the users.conf for the users module the sambashare user needs to look like the following:
defaultGroups: - adm - cdrom - dip - lpadmin - plugdev - name: sambashare must_exist: false system: true - sudo
For reference on the users.conf refer to the Calamares example