In the situations an Admin user with the 'sn_hr_core.admin' is unable to assign that role to another user. The following error may also get generated:
"Your role does not allow assignment of sn_hr_core.admin role"
In this instance, the reason for the error message and the scoped role not being assigned is because the Admin user was missing the scoped role named 'sn_hr_le_ent.admin'. This role is only assignable by 'assignable_by' itself which means you need to already have this role in order to be able to add the role to another user. This scoped role was found contained within 'sn_hr_le.admin'.
After adding the scoped role 'sn_hr_le_ent.admin' to the user record for the Admin, The admin user was able to add the 'sn_hr_core.admin' without any error message.