sbt cross build issue demo

build.sbt

lazy val root = project.in(file("."))
  .aggregate(core, moduleA, moduleB)
  .settings(
    crossScalaVersions := Nil
  )

lazy val core = project.in(file("core"))
  .settings(
     crossScalaVersions := Seq("2.11.12", "2.12.8")
  )

lazy val moduleA = project.in(file("moduleA"))
  .settings(
     crossScalaVersions := Seq("2.11.12", "2.12.8")
  )
  .dependsOn(core)

lazy val moduleB = project.in(file("moduleB"))
  .settings(
     crossScalaVersions := Seq("2.12.8")
  )
  .dependsOn(core)

Steps to reproduce:

1. run sbt
2. ++2.11.12 -v
3. compile