Incorrect behavior with type class derivation macro

CsvDerivation.scala

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context

trait Csv[A] { def apply(a: A): Seq[String] }
object Csv {
  implicit def deriveCsv[A]: Csv[A] = macro CsvMacro.csvImpl[A]

  implicit val csvStr: Csv[String] = new Csv[String] { def apply(s: String) = Seq(s) }
  implicit def csvSeq[A](implicit ca: Csv[A]): Csv[Seq[A]] = new Csv[Seq[A]] {
    def apply(a: Seq[A]) = a.flatMap(ca(_))
  }
}

object CsvMacro {
  def csvImpl[A: c.WeakTypeTag](c: Context): c.Expr[Csv[A]] = {
    import c.universe._

    val tpe = c.weakTypeOf[A]

    def fail = c.abort(c.enclosingPosition, s"Cannot derive Csv[$tpe]. Type is neither a sealed trait nor a case class.")

    println(s"Deriving Csv[$tpe]")

    if (tpe.typeSymbol.isClass) {
      val klass = tpe.typeSymbol.asClass
      if (klass.isCaseClass || (klass.isTrait && klass.isSealed)) {
        val accumulated = tpe.decls.collect {
          case m: MethodSymbol if m.isCaseAccessor =>
            q"_root_.scala.Predef.implicitly[Csv[${m.returnType.asSeenFrom(tpe, tpe.typeSymbol)}]].apply(x.${m.name})"
        }.reduceLeft((acc, x) => q"$acc ++ $x")

        c.Expr[Csv[A]](q"new Csv[$tpe] { def apply(x: $tpe): Seq[String] = $accumulated }")
      } else fail
    } else fail
  }
}

case class Foo(s: String)
case class TypeParam[A](a: A)
case class Wrapper(tp: TypeParam[Foo])

IncorrectBehavior.scala

// Derivation for Seq[Wrapper] has incorrect behavior
implicitly[Csv[Seq[Wrapper]]]
/*
Deriving Csv[Wrapper]
Deriving Csv[Seq[Wrapper]]
<console>:18: error: Cannot derive Csv[Seq[Wrapper]]. Type is neither a sealed trait nor a case class.
       implicitly[Csv[Seq[Wrapper]]]
*/



// Derivation for Seq[Wrapper] works correctly when there is an implicit Csv[Wrapper] in scope
implicit val w = implicitly[Csv[Wrapper]]
/*
Deriving Csv[Wrapper]
Deriving Csv[TypeParam[Foo]]
Deriving Csv[Foo]
w: CsvMacro.Csv[Wrapper] = $anon$1@712594f4
*/
implicitly[Csv[Seq[Wrapper]]]
/*
res1: CsvMacro.Csv[Seq[Wrapper]] = csv$$$Lambda$1644/1243911696@ebbbe20
*/

load.sh

#!/usr/bin/env sh

# To load the code in CsvDerivation.scala, run
#   $ curl -sL https://gist.github.com/mrdziuban/f3389368ab1cdeb7f8bd06c4b45e080c/raw/load.sh | bash
# Then run the commands shown in IncorrectBehavior.scala

test -e $HOME/.coursier/coursier || \
  (mkdir -p $HOME/.coursier && \
   curl -sL -o $HOME/.coursier/coursier https://git.io/vgvpD && \
   chmod +x $HOME/.coursier/coursier)

tmpfile="$(mktemp)"
curl -sL -o $tmpfile https://gist.github.com/mrdziuban/f3389368ab1cdeb7f8bd06c4b45e080c/raw/CsvDerivation.scala

$HOME/.coursier/coursier launch -q -M 'ammonite.Main' -P \
  com.lihaoyi:ammonite_2.12.6:1.1.2 \
  -- --predef $tmpfile < /dev/tty