Why does this Scala code type check even though it shouldn't?

IOMonadExercise.scala

object IOMonadExercise extends App {

  sealed trait IO[A]

  case class Return[A](value: A) extends IO[A]

  case class Suspend[A](f: () => A) extends IO[A]

  case class FlatMap[A, B](io: IO[A], cont: A => IO[B]) extends IO[B]

  object IO {
    def apply[A](a: => A): IO[A] = Suspend(() => a)
  }

  object Interpreter {
    def run[A](io: IO[A]): A = {
      io match {
        case Return(a) => a
        case Suspend(f) => f()

        case FlatMap(Return(a), cont) => run(cont(a))
        case FlatMap(Suspend(f), cont) => run(cont(f()))

        // this case compiles for whatever reason but shouldn't type check (k1 returns IO[B] and k2 expects just B)
        // accordingly, there is a ClassCastException in the runtime
        case FlatMap(FlatMap(io1, k1), k2) => run(FlatMap(io1, k1 andThen k2))

        // this case is the one that actually works
//        case FlatMap(FlatMap(io1, k1), k2) => run(flatten(io1, k1, k2))
      }
    }

    def flatten[A, B, C](io: IO[A], k1: A => IO[B], k2: B => IO[C]): FlatMap[A, C] = {
      FlatMap(io, a => FlatMap(k1(a), k2))
    }
  }


  def sum(i: Int): IO[Int] = {
    Stream.range(0, i).foldLeft(IO(0))((io, i) => FlatMap(io, (s: Int) => IO(s + i)))
  }

  val n = 100000
  val sumNIO: IO[Int] = sum(n)

  val sumN: Int = Interpreter.run(sumNIO)
  println(s"sum of 1..$n by IO loop  : $sumN")
  println(s"sum of 1..$n by math expr: ${n * (n - 1) / 2}")
  assert(sumN == n * (n - 1) / 2)
}