Рекурсивный список пустых полей с помощью Shapeless

Я пытаюсь рекурсивно перечислить пустые поля с помощью Shapeless. Но, похоже, не отображаются все поля:

https://scastie.scala-lang.org/PtLdSRC2Qfipu054Hzerrw

import shapeless._
import shapeless.labelled.FieldType

trait NullFields[T] {
  def apply(value: T, parent: String): Seq[String]
}

object NullFields extends NullFieldsLowPriority {
  implicit lazy val hNil = new NullFields[HNil] {
    override def apply(value: HNil, parent: String) = Seq.empty
  }

  implicit def hConsOption[K <: Symbol, V, TailFields <: HList](implicit
      witness: Witness.Aux[K],
      tailNullFields: NullFields[TailFields]
  ) = new NullFields[FieldType[K, Option[V]] :: TailFields] {
    override def apply(
        params: FieldType[K, Option[V]] :: TailFields,
        parent: String
    ) =
      (if (params.head.isEmpty) Seq(parent + witness.value.name)
       else Seq.empty) ++
        tailNullFields(params.tail, parent)
  }

  implicit def hConsCaseClass[
      K <: Symbol,
      CC <: Product,
      Fields <: HList,
      TailFields <: HList
  ](implicit
      generic: LabelledGeneric.Aux[CC, Fields],
      nullFields: NullFields[Fields],
      witness: Witness.Aux[K],
      tailNullFields: NullFields[TailFields]
  ) = new NullFields[FieldType[K, CC] :: TailFields] {
    override def apply(params: FieldType[K, CC] :: TailFields, parent: String) =
      nullFields(generic.to(params.head), s"$parent${witness.value.name}.") ++
        tailNullFields(params.tail, parent)
  }

  implicit def caseClass[CC, Fields <: HList](implicit
      generic: LabelledGeneric.Aux[CC, Fields],
      nullFields: NullFields[Fields]
  ) = new NullFields[CC] {
    override def apply(value: CC, parent: String) =
      nullFields(generic.to(value), parent)
  }
}

trait NullFieldsLowPriority {
  implicit def hConsDefault[K <: Symbol, V, TailFields <: HList](implicit
      tailNullFields: NullFields[TailFields]
  ) = new NullFields[FieldType[K, V] :: TailFields] {
    override def apply(params: FieldType[K, V] :: TailFields, parent: String) =
      tailNullFields(params.tail, parent)
  }
}

case class B(
    balba: Int = 0,
    profile_experience_desc: Option[String] = None,
    vmaf: Va
)
case class Va(
    tw_segment: Option[Double],
    i20_segment: Option[Double],
    ttq_segment: Option[Int]
)

println(implicitly[NullFields[B]].apply(B(vmaf = Va(None, None, None)), ""))