In case of subresources using the same "iri" of the parent, all fields are merged into the same resource.

Test case, two entities with the same iri:

// entities
User:
 id
 username
 password
 userProfile (OneToOne)
UserProfile:
 id
 photo
 firstname
 ...

// resources file
App\Entity\User:
 iri: 'http://schema.org/Person'
App\Entity\UserProfile:
 iri: 'http://schema.org/Person'

Result is all the fields of the latest entity (with the same iri) parsed by the parseHydraDocumentation(), in my case the fields are from "UserProfile" entity.