syntax like this
criteriaBuilder.treat(root, SubClass.class).fetch("subClassField", JoinType.LEFT);
is not working, since only joins are present here
https://github.com/hibernate/hibernate-orm/blob/9ff14a33c78b11f03351748dfe4a5610517325f3/hibernate-core/src/main/java/org/hibernate/query/criteria/internal/QueryStructure.java#L308

I tried to add

renderFetches( jpaqlQuery, renderingContext, treat.getFetches() );

to the next line and it seems to work.
Maybe it was intended and I am missing something?