With an entity hierarchy as souch:

A
B extends A
C1 extendes B
C2 extends B
D extends C1

creating a query using treat:

select A from A where treat(b as B).property = 'something';

the generated sql will be:

select ... 
from A
where DTYPE in ('C2') and ...

C1 and B are missing, but should be there, since they are not Abstract.

The culprit is this line of code:
https://github.com/hibernate/hibernate-orm/blob/6acfd17548ac1ccfd9af6404965a8719c94d705c/hibernate-core/src/main/java/org/hibernate/persister/entity/SingleTableEntityPersister.java#L635

    if ( !actualQueryable.hasSubclasses() ) {
        values.add( actualQueryable.getDiscriminatorSQLValue() );
    }

the subclass should be excluded from the query only if the entity is Abstract