Without the alias in the 2nd query, it’s unclear if the unaliased fields in the subquery refer to the SGBSTDN table that’s in the main query or in the subquery. It appears that without the alias, the unaliased fields in the subquery are referring to the SGBSTDN table in the subquery (and subject to the terms of that where clause) rather the one in the main query.
You really should use aliases when multiple tables are involved to prevent ambiguity (and make your queries easier to understand for people who don’t know which columns belong to which tables)