Doctrine doesn't understand when a query is built from an association to a SINGLE_TABLE parent to get its successors relations.
Many Lice reside in an Animal (Louse @ManyToOne Animal)
Animals can specialize into Cats or Dogs (SINGLE_TABLE Inheritance)
A Dog can have one Bone (Dog @OneToOne Bone)
A Cat can have one Yarn (Yarn @OneToOne Yarn)
From a specific Louse, Doctrine can't get the Animal's Bone or Yarn.