IncOptions.useOptimizedSealed is broken on 2.13

[smarter]

build.sbt:

lazy val root = (project in file(".")).
  settings(
    name := "sbt-hello",
    scalaVersion := "2.13.11"
  )
ThisBuild / incOptions ~= { _.withUseOptimizedSealed(true) }

Sealed.scala:

sealed trait Sealed
class Child1 extends Sealed

PatMat.scala:

object PatMat {
  def test(x: Sealed): Int = x match {
    case _: Child1 => 1
  }
}

1. compile
2. Add an extra child in Sealed.scala: class Child2 extends Sealed
3. compile again, only Sealed.scala is recompiled.

If I change scalaVersion to 2.12.18, then PatMat.scala is recompiled as expected.
I assume the zinc scripted tests such as https://github.com/sbt/zinc/tree/develop/zinc/src/sbt-test/source-dependencies/patMat-scope only run with 2.12, it'd be great to run them with 2.13 too to catch this sort of issues.

/cc @retronym who might be aware of projects that use useOptimizedSealed.

[lrytz]

Fixed in #1278