Today the export/import of UserGroups and Roles does not contain the
assignments between roles and user groups. So when
"transporting" the user groups and roles from one stage (or
backup) to another stage (new system) the formerly configured
assignments are lost.
A support ticket (https://help.liferay.com/hc/en-us/requests/117859)
confirms this issue and recommended to open a feature request.
