User is unable to dot-walk from a table which is likely to be sharded (sys_email, sys_audit, etc). For example, the email field on the sys_email table is a reference field, however a user cannot expand email to get to any of the dot walk fields.
The restriction placed here is at the source code level. A user will not be able to dot-walk to any table which is sharded.
This is expected behavior. Users are not able to dot-walk to fields on a table that is sharded.